libgui|Refactor|GLTarget: Extra attachments in addition to texture

Rewrote and cleaned up some of the implementation of GLTarget to
account for the fact that one may need supporting attachments in
addition to a texture.

Thanks to cmbruns for pointing this out.
Fixed|Client: Crash due to accessing OpenGL from non-UI thread

During busy mode, if the value of 'rend-vr-mode' was updated due to
restoring a saved config, it meant that the root widget was resized
from a non-UI thread.

Now the resizing is deferred to the beginning of the next frame.