graphics: Call glFlush after glTexSubImage2D for MacBook Pro

internal/graphics/command.go

@@ -149,7 +149,17 @@ func (q *commandQueue) Flush() error {
 		}
 		numc := len(g)
 		indexOffsetInBytes := 0
+		var lastC command
 		for _, c := range g {
+			// On MacBook Pro (2013 Late?), glTexSubImage2D might not be finished
+			// before the next different comand. Call glFlush explicitly.
+			if _, ok := lastC.(*replacePixelsCommand); ok {
+				if _, ok := c.(*replacePixelsCommand); !ok {
+					opengl.GetContext().Flush()
+				}
+			}
+			lastC = c
+
 			if err := c.Exec(indexOffsetInBytes); err != nil {
 				return err
 			}