win32u: Optimize blend_argb function when source image is opaque
Wine-Bug: https://bugs.winehq.org/show_bug.cgi?id=53947
To check the performance:
- Run the game and notice the sluggish of screeen scrolling in title screen
There are also difference in profiler results: ccfad279 (comment 56119)
Edited by Bartosz Kosiorek