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
::stable-branch::merge=121152f1632170629122559482a298ea8b3aaf75
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
::stable-branch::merge=28e02ce5a65a4eb60187acc1bca5e493d7cb78f9
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
::stable-branch::merge=244ac040c5cb901177e319dea3fab7891e2bd6f3
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
::stable-branch::merge=0275b3fc9bbf9bea9dd5b7740ba34295a7514424
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
::stable-branch::merge=c0eb3105f4f3c4daf3f2025246c516a8195686b7