Flipfuncpointers (take two)

See merge request STJr/SRB2!864

The rule for PACKETVERSION is too confusing, it's also too hard to unify formats

See merge request STJr/SRB2!875

srb2:// server URLs

See merge request STJr/SRB2!858

Fix MD2 rollangle in reverse gravity

See merge request STJr/SRB2!818

OGL horizon lines

See merge request STJr/SRB2!861

ExecCfg fixes

See merge request STJr/SRB2!821

Hide record & NiGHTS attack options if there are no available maps for them.

See merge request STJr/SRB2!868

Allow emblem hints (and radar) in record attack.

See merge request STJr/SRB2!820

Florida man fixes sectors with light level 256 blacking out OpenGL objects; mappers riot.

See merge request STJr/SRB2!840

Only initialise OpenGL if the user intends to load it (for real this time)

See merge request STJr/SRB2!796

Fix console backpic

See merge request STJr/SRB2!850

Turn FIXED_TO_FLOAT into a function

See merge request STJr/SRB2!869

Fix disconnected players being kicked for high ping

See merge request STJr/SRB2!866

use ATTRINLINE rather than inline to match some of the other functions in this file, though I'm told modern compilers ignore inlining info as they actually decide themselves now

(though I could pretend we still support the older ones lol)

Turn the fixed-float convert macros into inline functions, like those in GZDoom. The old macros get to be just the same as calling these inline functions.

* Replace the texheight parameter provided directly to it with a previously existing (now renamed) global used for the same purpose, so that it can be used as an interchangeable function pointer with R_DrawMaskedColumn.
* Using the above, optimise R_DrawVisSprite to call a function pointer in a tighter loop rather than check SC_VFLIP each time around.
* SHORT macro can involve repeated operations; calculate once and put in memory for both RANGECHECK and papersprite.
* Remove irrelevant range check (already covered by existing range check immediately above) from R_DrawFlippedMaskedColumn and R_DrawMaskedColumn.
* "Warning: avoiding a crash in %s %d" is a terrible error message, and it chips away at the tightness of the loop just for something most people will never see printed. Replace with a PARANOIA I_Error in case someone actively wants to go hunting for its cause.

Updates and fixes for the ZB config

See merge request STJr/SRB2!859

Skip empty gametypes in level select menu

See merge request STJr/SRB2!819

Remove the multi-file ZB configs. We're stick with the single-file config for ZB and doing all the UDMF stuff in UDB