Allow for a more modular renderer backends (#990)

* Draft GPU3D renderer modularization

* Update sources C++ standard to C++17

The top-level `CMakeLists.txt` is already using the C++17 standard.

* Move GLCompositor into class type

Some other misc fixes to push towards better modularity

* Make renderer-implementation types move-only

These types are going to be holding onto handles
of GPU-side resources and shouldn't ever be copied around.

* Fix OSX: Remove 'register' storage class specifier

`register` has been removed in C++17...
But this keyword hasn't done anything in years anyways.

OSX builds consider this "warning" an error and it
stops the whole build.

* Add RestartFrame to Renderer3D interface

* Move Accelerated property to Renderer3D interface

There are points in the code base where we do:
`renderer != 0` to know if we are feeding
an openGL renderer. Rather than that we can instead just have this be
a property of the renderer itself.
With this pattern a renderer can just say how it wants its data to come
in rather than have everyone know that they're talking to an OpenGL
renderer.

* Remove Accelerated flag from GPU

* Move 2D_Soft interface in separate header

Also make the current 2D engine an "owned" unique_ptr.

* Update alignment attribute to standard alignas

Uses standardized `alignas` rather than compiler-specific
attributes.

https://en.cppreference.com/w/cpp/language/alignas

* Fix Clang: alignas specifier

Alignment must be specified before the array to align the entire array.

https://en.cppreference.com/w/cpp/language/alignas

* Converted Renderer3D Accelerated to variable

This flag is checked a lot during scanline rasterization. So rather
than having an expensive vtable-lookup call during mainline rendering
code, it is now a public constant bool type that is written to only once
during Renderer3D initialization.

src/frontend/qt_sdl/main.cpp

@@ -1013,7 +1013,7 @@ void ScreenPanelGL::paintGL()
         if (GPU::Renderer != 0)
         {
             // hardware-accelerated render
-            GPU::GLCompositor::BindOutputTexture(frontbuf);
+            GPU::CurGLCompositor->BindOutputTexture(frontbuf);
         }
         else
     #endif
@@ -2536,9 +2536,15 @@ int main(int argc, char** argv)
 
     Config::Load();
 
-#define SANITIZE(var, min, max)  { if (var < min) var = min; else if (var > max) var = max; }
+#define SANITIZE(var, min, max)  { var = std::clamp(var, min, max); }
     SANITIZE(Config::ConsoleType, 0, 1);
-    SANITIZE(Config::_3DRenderer, 0, 1);
+    SANITIZE(Config::_3DRenderer,
+    0,
+    0 // Minimum, Software renderer
+    #ifdef OGLRENDERER_ENABLED
+    + 1 // OpenGL Renderer
+    #endif
+    );
     SANITIZE(Config::ScreenVSyncInterval, 1, 20);
     SANITIZE(Config::GL_ScaleFactor, 1, 16);
     SANITIZE(Config::AudioVolume, 0, 256);