cairooverlay: Optimize premultiplication/unpremultiplication loops

Pull in video frame fields into local variables. Without this the
compiler must assume that they could've changed on every use and read
them from memory again.

This reduces the inner loop from 6 memory reads per pixels to 4, and the
number of writes stays at 3.
4 jobs for cairooverlay-premultiply in 5 minutes and 31 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Preparation
passed gst indent #59024

00:00:14

passed manifest #59023

00:00:17

 
  Build
passed build fedora x86_64 #59025

00:01:23

 
  Full Builds
passed build android arm64 api28 #59026

00:03:51