Use Qt provided display with GLX functions for consistency.
Remove unneeded assignment from previous code.
Delete per window GL context for Cocoa and just swap the view instead

This unfortunately re-introduces undesirable rendering behaviour for
slow renderers (e.g. first gen Intel HD graphics/Apple software
renderer) when the property window is open, but fixes property window
preview rendering for sufficiently fast renderers
Use bzalloc instead of bmalloc+memset
This commit contains more than I intended. Read below for more info.

1) Fixed the preview window. It now correctly displays the source.
2) The GLX backend now correctly uses the devices current swap.
3) We now set device->cur_swap to a default so we don't have to check it in every function.
4) Minor syntactical cleanups and perhaps some messiness added.
Removed no longer valid comment
Merge branch 'master' of https://github.com/jp9000/obs-studio