Fix using read color buffer for glReadPixels validation.

Change-Id: Ib153f6af75c982fae0325a104da3c0a4fc9ee9dc
Reviewed-on: https://swiftshader-review.googlesource.com/14548
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
diff --git a/src/OpenGL/libGLESv2/utilities.cpp b/src/OpenGL/libGLESv2/utilities.cpp
index 1cc650c..612926a 100644
--- a/src/OpenGL/libGLESv2/utilities.cpp
+++ b/src/OpenGL/libGLESv2/utilities.cpp
@@ -641,7 +641,7 @@
 			}
 		}
 
-		Renderbuffer *colorbuffer = framebuffer->getColorbuffer(0);
+		Renderbuffer *colorbuffer = framebuffer->getReadColorbuffer();
 
 		if(!colorbuffer)
 		{