Add support for falling back to glBlendEquationEXT if necessary.

This should help keep LÖVE from crashing on old cards with crappy drivers.

src/modules/graphics/Graphics.cpp

@@ -156,6 +156,7 @@ namespace graphics
 		{ "canvas", Graphics::SUPPORT_CANVAS },
 		{ "pixeleffect", Graphics::SUPPORT_PIXELEFFECT },
 		{ "npot", Graphics::SUPPORT_NPOT },
+		{ "subtractive", Graphics::SUPPORT_SUBTRACTIVE },
 	};
 
 	StringMap<Graphics::Support, Graphics::SUPPORT_MAX_ENUM> Graphics::support(Graphics::supportEntries, sizeof(Graphics::supportEntries));

src/modules/graphics/Graphics.h

@@ -85,6 +85,7 @@ namespace graphics
 			SUPPORT_CANVAS = 1,
 			SUPPORT_PIXELEFFECT,
 			SUPPORT_NPOT,
+			SUPPORT_SUBTRACTIVE,
 			SUPPORT_MAX_ENUM
 		};
 

src/modules/graphics/opengl/Graphics.cpp

@@ -511,10 +511,24 @@ namespace opengl
 	{
 		glAlphaFunc(GL_GEQUAL, 0);
 
-		if (mode == BLEND_SUBTRACTIVE)
-			glBlendEquation(GL_FUNC_REVERSE_SUBTRACT);
-		else
-			glBlendEquation(GL_FUNC_ADD);
+		if (GLEE_VERSION_1_4 || GLEE_ARB_imaging) {
+			if (mode == BLEND_SUBTRACTIVE) {
+				glBlendEquation(GL_FUNC_REVERSE_SUBTRACT);
+			} else {
+				glBlendEquation(GL_FUNC_ADD);
+			}
+		} else if (GLEE_EXT_blend_minmax && GLEE_EXT_blend_subtract) {
+			if (mode == BLEND_SUBTRACTIVE) {
+				glBlendEquationEXT(GL_FUNC_REVERSE_SUBTRACT_EXT);
+			} else {
+				glBlendEquationEXT(GL_FUNC_ADD_EXT);
+			}
+		} else {
+			if (mode == BLEND_SUBTRACTIVE) {
+				std::cerr << "This graphics card does not support the subtract blend mode!" << std::endl;
+			}
+			// GL_FUNC_ADD is the default even without access to glBlendEquation, so that'll still work.
+		}
 
 		if (mode == BLEND_ALPHA)
 			glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

src/modules/graphics/opengl/wrap_Graphics.cpp

@@ -19,6 +19,7 @@
 **/
 
 #include "wrap_Graphics.h"
+#include "GLee.h"
 #include <graphics/DrawQable.h>
 #include <image/ImageData.h>
 #include <font/Rasterizer.h>
@@ -793,6 +794,9 @@ namespace opengl
 					if (!Image::hasNpot())
 						supported = false;
 					break;
+				case Graphics::SUPPORT_SUBTRACTIVE:
+					supported = (GLEE_VERSION_1_4 || GLEE_ARB_imaging) || (GLEE_EXT_blend_minmax && GLEE_EXT_blend_subtract);
+					break;
 				default:
 					supported = false;
 			}