Fix GLES/Emscripten rendering issues

[kblaschke]

In GLES builds, the current master code produces some OpenGL errors after loading additional presets or doing a transition, e.g. making the screen turn green or start flickering.

It needs to be investigated if there's an issue with setting up the framebuffers and attachments, or if there are any resources not allocated or freed properly.

[kblaschke]

Fixes are now properly merged. If anyone checked out/pulled master in the past eleven hours, or has a fork with auto sync enabled, please make sure to reset your master branch to the state of this repository via git reset - do not use pull again in this case, as your local/forked master branch history will then be broken.