OpenGL/ES: Fix 'R' and 'B' bits inverted on all platforms

Balló György requested to merge City-busz/gtk:gles-swapped-colors into gtk-3-24

The color channels are swapped on Linux too, not only on Windows. It can be reproduced by running the "OpenGL Area" example from gtk3-demo with GDK_GL=gles, or play a video in totem with GDK_GL=gles.

Fixes #3032 (closed)

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