metal: fix uniform data corruption

The wrong buffer binding offset was being used in 2/3rds of frames.

src/modules/graphics/metal/Graphics.h

@@ -242,6 +242,7 @@ private:
 	StreamBuffer *uniformBuffer;
 	StreamBuffer::MapInfo uniformBufferData;
 	size_t uniformBufferOffset;
+	size_t uniformBufferGPUStart;
 
 	Buffer *defaultAttributesBuffer;