Support MIN/MAX blend on OpenGL + ES

src/render/opengl/SDL_render_gl.c

@@ -374,6 +374,10 @@ static GLenum GetBlendEquation(SDL_BlendOperation operation)
         return GL_FUNC_SUBTRACT;
     case SDL_BLENDOPERATION_REV_SUBTRACT:
         return GL_FUNC_REVERSE_SUBTRACT;
+    case SDL_BLENDOPERATION_MINIMUM:
+        return GL_MIN;
+    case SDL_BLENDOPERATION_MAXIMUM:
+        return GL_MAX;
     default:
         return GL_INVALID_ENUM;
     }