OpenGL new clipping backport

See merge request STJr/SRB2!315

Compiling cleanup and fixes

See merge request STJr/SRB2!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.

Some code cleanup.

See merge request STJr/SRB2!298

Winsock error handling fixes

See merge request STJr/SRB2!303

* Move the misc types in the file to bottom, so that ATTRPACK at least is usable for RGBA_t
* Include endian.h, so that UINT2RGBA can be defined correctly for big endian builds
* Add more comments to make clear the main sections of the file

Remove FUNCMATH from functions with a void return value or args, or examine variables other than their args (which could be modified)

D_PostEvent_end is only used by Allegro (used by the DOS port) to help timers work, so check for PC_DOS in preprocessor code. Also remove FUNCMATH from said function.

P_NullPrecipThinker no longer should have FUNCMATH (though I'm not sure if it should have had it in the first place anyway)

File loading hotfix

See merge request STJr/SRB2!310

Make sure handles of files opened by W_LoadWadFile are closed if we abort loading the files for whatever reason.

ESPECIALLY if the file is already loaded in SRB2, that's just silly.

Patch offsets fix

See merge request STJr/SRB2!305

MI minor refactoring

See merge request STJr/SRB2!294

Multi-patch texture support for transparency AND translucency

See merge request STJr/SRB2!293

now that V_FLIP's effects are always supported in V_DrawFixedPatch, there's no need for the "flip" variable anymore

HWR_DrawFixedPatch: Add V_OFFSET support for V_FLIP in hardware code as well, also add missing SHORTs for gpatch fields