UPBGE: Fix frame buffer copy shader.

Do not mix usage of gl_FragData and gl_FragColor. The frame buffer shader now
only uses gl_FragData for the first slot, others slots are ignored as they
are never used after the frame buffer is copied, filters use the original
frame buffer (or the one blitted) which contains all the slots.

Fix issue: #969.

Author: panzergame

source/blender/gpu/shaders/gpu_shader_frame_buffer_frag.glsl

@@ -38,19 +38,19 @@ void main()
 	if (stippleid == STIPPLE_ROW) {
 		int result = int(mod(gl_FragCoord.y, 2));
 		if (result != 0) {
-			gl_FragColor = texture2D(lefteyetex, co);
+			gl_FragData[0] = texture2D(lefteyetex, co);
 		}
 		else {
-			gl_FragColor = texture2D(righteyetex, co);
+			gl_FragData[0] = texture2D(righteyetex, co);
 		}
 	}
 	else if (stippleid == STIPPLE_COLUMN) {
 		int result = int(mod(gl_FragCoord.x, 2));
 		if (result == 0) {
-			gl_FragColor = texture2D(lefteyetex, co);
+			gl_FragData[0] = texture2D(lefteyetex, co);
 		}
 		else {
-			gl_FragColor = texture2D(righteyetex, co);
+			gl_FragData[0] = texture2D(righteyetex, co);
 		}
 	}
 #elif defined(ANAGLYPH)
@@ -63,6 +63,6 @@ void main()
 #endif
 
 #ifdef COLOR_MANAGEMENT
-	linearrgb_to_srgb(gl_FragColor, gl_FragColor);
+	linearrgb_to_srgb(gl_FragData[0], gl_FragData[0]);
 #endif
 }