Revert "Windows now get mouseover events when not active (not entirely robust)"

This reverts commit d371fbce4fe11efd831af1ed3a39139aec8f53a9.
Revert "Cleaned up texture memory management: textures are now shared across OpenGL contexts, and gui_textures and gui_resources are handled globally rather than per-window"

This reverts commit 44b402075b55b732edaf783cf228a08d4cadca34.