Implement GPU-side corona fading

Uses GL_ARB_query_buffer_object to retrieve query results directly into a
buffer. This allows corona fading to be moved into the fragment shader,
preventing a round-trip to the CPU and synchronous rendering. Depending on the
machine's configuration, it can give a dramatic performance boost.

From: Alex Goins <agoins@nvidia.com>

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@12094 d7cf8633-e32d-0410-b094-e92efae38249
Cache corona occlusion buffer

Rather than creating and destroying the occlusion buffer every frame, cache it.
This improves performance slightly.

From: Alex Goins <agoins@nvidia.com>

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@12095 d7cf8633-e32d-0410-b094-e92efae38249
SDL2: Make work numeric keypad keys when numlock is off, K_PRINTSCREEN,  K_SCROLLOCK and K_NUMLOCK. Also fix a debug print (event.key.keysym.unicode doesn't exist anymore in SDL2)

From: terencehill <piuntn@gmail.com>

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@12096 d7cf8633-e32d-0410-b094-e92efae38249
Implement proper support for utf8 characters in SDL2

From: terencehill <piuntn@gmail.com>

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@12097 d7cf8633-e32d-0410-b094-e92efae38249
If vid_desktopfullscreen is set, manually initialize window with the current desktop resolution instead of with the default values and then letting sdl correct it automatically when they differ. It caused issues switching from fullscreen to window mode in the first game run

Signed-off-by: terencehill <piuntn@gmail.com>

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@12098 d7cf8633-e32d-0410-b094-e92efae38249