Camera subsector crash fix

See merge request STJr/SRB2!318

OpenGL HUD scaling fix

See merge request STJr/SRB2!320

Fix desynch when toggling analog mode or flipcam

Update SDL Mixer to 2.0.4

See merge request STJr/SRB2!322

Update SDL2 to 2.0.9

See merge request STJr/SRB2!321

Remove unnecessary SDLCALL from IMG_xpm.c declarations

See merge request STJr/SRB2!327

Missed #ifdef HAVE_LIBGME in win_snd

See merge request STJr/SRB2!326

mixer_sound fix x64 build issue

See merge request STJr/SRB2!323

Save the current renderer to screenshots.

See merge request STJr/SRB2!324

Fix OpenGL's HUD not being scaled the same way as Software's, by removing an old OpenGL-specific hack for fdupx/fdupy

Fix for object z positions on floating, bobbing FOFs

See merge request STJr/SRB2!316

Split camera initialisation code from P_SetupLevel, make sure the camera has an initial subsector set!

Fix monitors (and other objects) on a floating, bobbing FOF being stuck slightly above said FOF after it has finished bobbing

OpenGL new clipping backport

See merge request STJr/SRB2!315

Alias recursion hotfix

See merge request STJr/SRB2!314

Compiling cleanup and fixes

See merge request STJr/SRB2!311