Implement if control flow for PP
The X11 glamor compositing uses quite many if's in its fragment shader, and they're too complex to be lowered to select operation.
Here's an example, when I'm trying to run gtk3-demo
: https://pastebin.aosc.io/paste/cdDhRbnpSdKrkEmiY2wOdA
Implementing if may be able to make glamor working with general 2D programs.
P.S. should we switch to use fd.o bugzilla for issues on Lima?