VideoCommon: Rework scissor handling

This increases accuracy, fixing the white rendering in Major Minor's Majestic March.  However, the hardware backends can only have one viewport and scissor rectangle at a time, while sometimes multiple are needed to accurately emulate what is happening.  If possible, this will need to be fixed later.

Source/Core/VideoCommon/BPFunctions.h

@@ -7,16 +7,131 @@
 
 #pragma once
 
-#include "Common/MathUtil.h"
+#include <utility>
+#include <vector>
 
-struct BPCmd;
+#include "Common/MathUtil.h"
+#include "VideoCommon/BPMemory.h"
+struct XFMemory;
 
 namespace BPFunctions
 {
+struct ScissorRange
+{
+  constexpr ScissorRange() = default;
+  constexpr ScissorRange(int offset, int start, int end) : offset(offset), start(start), end(end) {}
+  int offset = 0;
+  int start = 0;
+  int end = 0;
+};
+
+struct ScissorRect
+{
+  constexpr ScissorRect(ScissorRange x_range, ScissorRange y_range)
+      :  // Rectangle ctor takes x0, y0, x1, y1.
+        rect(x_range.start, y_range.start, x_range.end, y_range.end), x_off(x_range.offset),
+        y_off(y_range.offset)
+  {
+  }
+
+  MathUtil::Rectangle<int> rect;
+  int x_off;
+  int y_off;
+
+  int GetArea() const;
+};
+
+// Although the GameCube/Wii have only one scissor configuration and only one viewport
+// configuration, some values can result in multiple parts of the screen being updated.
+// This can happen if the scissor offset combined with the bottom or right coordinate ends up
+// exceeding 1024; then, both sides of the screen will be drawn to, while the middle is not.
+// Major Minor's Majestic March causes this to happen during loading screens and other scrolling
+// effects, though it draws on top of one of them.
+// This can also happen if the scissor rectangle is particularly large, but this will usually
+// involve drawing content outside of the viewport, which Dolphin does not currently handle.
+//
+// The hardware backends can currently only use one viewport and scissor rectangle, so we need to
+// pick the "best" rectangle based on how much of the viewport would be rendered to the screen.
+// If we choose the wrong one, then content might not actually show up when the game is expecting it
+// to.  This does happen on Major Minor's Majestic March for the final few frames of the horizontal
+// scrolling animation, but it isn't that important.  Note that the assumption that a "best"
+// rectangle exists is based on games only wanting to draw one rectangle, and accidentally
+// configuring the scissor offset and size of the scissor rectangle such that multiple show up;
+// there are no known games where this is not the case.
+struct ScissorResult
+{
+  ScissorResult(const BPMemory& bpmem, const XFMemory& xfmem);
+  ~ScissorResult() = default;
+  ScissorResult(const ScissorResult& other)
+      : scissor_tl{.hex = other.scissor_tl.hex}, scissor_br{.hex = other.scissor_br.hex},
+        scissor_off{.hex = other.scissor_off.hex}, viewport_left{other.viewport_left},
+        viewport_right{other.viewport_right}, viewport_top{other.viewport_top},
+        viewport_bottom{other.viewport_bottom}, m_result{other.m_result}
+  {
+  }
+  ScissorResult& operator=(const ScissorResult& other)
+  {
+    if (this == &other)
+      return *this;
+    scissor_tl.hex = other.scissor_tl.hex;
+    scissor_br.hex = other.scissor_br.hex;
+    scissor_off.hex = other.scissor_off.hex;
+    viewport_left = other.viewport_left;
+    viewport_right = other.viewport_right;
+    viewport_top = other.viewport_top;
+    viewport_bottom = other.viewport_bottom;
+    m_result = other.m_result;
+    return *this;
+  }
+  ScissorResult(ScissorResult&& other)
+      : scissor_tl{.hex = other.scissor_tl.hex}, scissor_br{.hex = other.scissor_br.hex},
+        scissor_off{.hex = other.scissor_off.hex}, viewport_left{other.viewport_left},
+        viewport_right{other.viewport_right}, viewport_top{other.viewport_top},
+        viewport_bottom{other.viewport_bottom}, m_result{std::move(other.m_result)}
+  {
+  }
+  ScissorResult& operator=(ScissorResult&& other)
+  {
+    if (this == &other)
+      return *this;
+    scissor_tl.hex = other.scissor_tl.hex;
+    scissor_br.hex = other.scissor_br.hex;
+    scissor_off.hex = other.scissor_off.hex;
+    viewport_left = other.viewport_left;
+    viewport_right = other.viewport_right;
+    viewport_top = other.viewport_top;
+    viewport_bottom = other.viewport_bottom;
+    m_result = std::move(other.m_result);
+    return *this;
+  }
+
+  // Input values, for use in statistics
+  ScissorPos scissor_tl;
+  ScissorPos scissor_br;
+  ScissorOffset scissor_off;
+  float viewport_left;
+  float viewport_right;
+  float viewport_top;
+  float viewport_bottom;
+
+  // Actual result
+  std::vector<ScissorRect> m_result;
+
+  ScissorRect Best() const;
+
+private:
+  ScissorResult(const BPMemory& bpmem, std::pair<float, float> viewport_x,
+                std::pair<float, float> viewport_y);
+
+  int GetViewportArea(const ScissorRect& rect) const;
+  bool IsWorse(const ScissorRect& lhs, const ScissorRect& rhs) const;
+};
+
+ScissorResult ComputeScissorRects();
+
 void FlushPipeline();
 void SetGenerationMode();
-void SetScissor();
-void SetViewport();
+void SetScissorAndViewport();
 void SetDepthMode();
 void SetBlendMode();
 void ClearScreen(const MathUtil::Rectangle<int>& rc);