[vobsub] Fix for incorrect colours with vobsubs and GLES

Fixes #15146

The code provides the overlay texture in BGRA format, which it informs GL about.
However GLES does not support BGRA, and just claims it is RGBA which results in a red/blue swap.

So this code path needs a red/blue swap.
Merge pull request #5450 from popcornmix/vobsub_bgr

[vobsub] Fix for incorrect colours with vobsubs and GLES