VideoCommon: add custom pixel shader definition and custom shader header to shadergen common as it will be used by both the special and uber shader variant of pixel shaders

Source/Core/VideoCommon/ShaderGenCommon.h

@@ -7,6 +7,7 @@
 #include <functional>
 #include <iterator>
 #include <string>
+#include <string_view>
 #include <type_traits>
 #include <vector>
 
@@ -327,3 +328,21 @@ static const char s_geometry_shader_uniforms[] = "\tfloat4 " I_STEREOPARAMS ";\n
                                                  "\tfloat4 " I_LINEPTPARAMS ";\n"
                                                  "\tint4 " I_TEXOFFSET ";\n"
                                                  "\tuint vs_expand;\n";
+
+constexpr std::string_view CUSTOM_PIXELSHADER_COLOR_FUNC = "customShaderColor";
+
+struct CustomPixelShader
+{
+  std::string custom_shader;
+
+  bool operator==(const CustomPixelShader& other) const = default;
+};
+
+struct CustomPixelShaderContents
+{
+  std::vector<CustomPixelShader> shaders;
+
+  bool operator==(const CustomPixelShaderContents& other) const = default;
+};
+
+void WriteCustomShaderStructDef(ShaderCode* out, u32 numtexgens);