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

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

OpenGL new clipping backport

See merge request STJr/SRB2!315

Compiling cleanup and fixes

See merge request !311

Made some efforts to improve efficiency of new code, hard to tell if I've made it better or worse though honestly

R_IsEmptyLine is now a thing too btw

More progress, NEWCLIP added to doomdef.h, sadly it actually all lags the game so I've disabled it for now

Other notes:
* on second thought I'll keep the hw_clip functions' gld prefixes rather than HWR, not like it matters either way
* despite the extra lag it does fix the issues with translucent walls and such when displayed at different vertical angles, such as with the GFZ1 waterfall

Other notes:
* Renamed all new functions to have HWR_ prefix instead of gld_, for consistency
* HWR_FrustrumSetup and HWR_SphereInFrustum are disabled and require HAVE_SPHEREFRUSTRUM. This is because 1) SRB2CB did not need the code, so presumably neither will we, and 2) there are some OpenGL API functions used there that due to our way of using OpenGL we don't use outside of r_opengl.c, which makes dealing with HWR_FrustrumSetup complicated in theory
* The new clipping functions are not added to OpenGL's "main" rendering code itself just yet, they're just available to use now once hw_clip.h is included

Opengl improvements backport

See merge request STJr/SRB2!304

Update readme

See merge request STJr/SRB2!313

Update copyright info

See merge request STJr/SRB2!309

Minor remote viewpoint bugfixes.