Image draw: Fix/workaround image corruption on draw

Was mainly visible with high-res image. Not entirely clear why it is
only happening on macOS. While the entire function should be re-written
to make float images displayed faster, still nice to fix the drawing.

Reviewers: fclem

Reviewed By: fclem

Differential Revision: https://developer.blender.org/D4664

source/blender/editors/screen/glutil.c

@@ -286,6 +286,13 @@ void immDrawPixelsTexScaled_clipping(IMMDrawPixelsTexState *state,
 			immAttr2f(texco, (float)(0 + offset_left) / tex_w, (float)(subpart_h - offset_top) / tex_h);
 			immVertex2f(pos, rast_x + (float)offset_left * xzoom, rast_y + (float)(subpart_h - offset_top) * yzoom * scaleY);
 			immEnd();
+
+			/* NOTE: Weirdly enough this is only required on macOS. Without this there is some sort of
+			 * bleeding of data is happening from tiles which are drawn later on.
+			 * This doesn't seem to be too slow, but still would be nice to have fast and nice solution. */
+#ifdef __APPLE__
+			glFlush();
+#endif
 		}
 	}