[GPU] Start texture combiner implementation

This commit first implements experimental and incomplete
texture combiner support. Currently only the first texture combiner
is implemented. Many sources and combine modes are not implemented yet.

include/renderer_gl/renderer_gl.hpp

@@ -24,7 +24,14 @@ class Renderer {
 	OpenGL::VertexBuffer vbo;
 	GLint alphaControlLoc = -1;
 	GLint texUnitConfigLoc = -1;
-	
+
+	// TEV configuration uniform locations
+	GLint textureEnv0SourceLoc = -1;
+	GLint textureEnv0OperandLoc = -1;
+	GLint textureEnv0CombinerLoc = -1;
+	GLint textureEnv0ColorLoc = -1;
+	GLint textureEnv0ScaleLoc = -1;
+
 	// Depth configuration uniform locations
 	GLint depthOffsetLoc = -1;
 	GLint depthScaleLoc = -1;