427a764: subtractive blend (OpenGL only for now) [Glenn Maynard]

I was looking into writing the equivalent Direct3D code; still not 100% sure if my code is right, so it stays local for now.

src/RageDisplay_OGL.cpp

@@ -1715,6 +1715,8 @@ void RageDisplay_OGL::SetBlendMode( BlendMode mode )
 	{
 		if( mode == BLEND_INVERT_DEST )
 			GLExt.glBlendEquation( GL_FUNC_SUBTRACT );
+		else if( mode == BLEND_SUBTRACT )
+			GLExt.glBlendEquation( GL_FUNC_REVERSE_SUBTRACT );
 		else
 			GLExt.glBlendEquation( GL_FUNC_ADD );
 	}
@@ -1729,6 +1731,9 @@ void RageDisplay_OGL::SetBlendMode( BlendMode mode )
 	case BLEND_ADD:
 		iSourceRGB = GL_SRC_ALPHA; iDestRGB = GL_ONE;
 		break;
+	case BLEND_SUBTRACT:
+		iSourceRGB = GL_SRC_ALPHA; iDestRGB = GL_ONE_MINUS_SRC_ALPHA;
+		break;
 	case BLEND_MODULATE:
 		iSourceRGB = GL_ZERO; iDestRGB = GL_SRC_COLOR;
 		break;

src/RageTypes.cpp

@@ -74,6 +74,7 @@ static const char *BlendModeNames[] =
 {
 	"Normal",
 	"Add",
+	"Subtract",
 	"Modulate",
 
 	/*

src/RageTypes.h

@@ -9,6 +9,7 @@ enum BlendMode
 {
 	BLEND_NORMAL,
 	BLEND_ADD,
+	BLEND_SUBTRACT,
 	BLEND_MODULATE,
 	BLEND_COPY_SRC,
 	BLEND_ALPHA_MASK,