maxwell_3d: Implement alternate blend equations.

- Used by Undertale.

src/video_core/engines/maxwell_3d.h

@@ -389,6 +389,13 @@ public:
                 ReverseSubtract = 3,
                 Min = 4,
                 Max = 5,
+
+                // These values are used by Nouveau and some games.
+                AddGL = 0x8006,
+                SubtractGL = 0x8007,
+                ReverseSubtractGL = 0x8008,
+                MinGL = 0x800a,
+                MaxGL = 0x800b
             };
 
             enum class Factor : u32 {

src/video_core/renderer_opengl/maxwell_to_gl.h

@@ -218,14 +218,19 @@ inline GLenum DepthCompareFunc(Tegra::Texture::DepthCompareFunc func) {
 inline GLenum BlendEquation(Maxwell::Blend::Equation equation) {
     switch (equation) {
     case Maxwell::Blend::Equation::Add:
+    case Maxwell::Blend::Equation::AddGL:
         return GL_FUNC_ADD;
     case Maxwell::Blend::Equation::Subtract:
+    case Maxwell::Blend::Equation::SubtractGL:
         return GL_FUNC_SUBTRACT;
     case Maxwell::Blend::Equation::ReverseSubtract:
+    case Maxwell::Blend::Equation::ReverseSubtractGL:
         return GL_FUNC_REVERSE_SUBTRACT;
     case Maxwell::Blend::Equation::Min:
+    case Maxwell::Blend::Equation::MinGL:
         return GL_MIN;
     case Maxwell::Blend::Equation::Max:
+    case Maxwell::Blend::Equation::MaxGL:
         return GL_MAX;
     }
     LOG_ERROR(Render_OpenGL, "Unimplemented blend equation={}", static_cast<u32>(equation));