Fix outdated sky wall code in shader branch

See merge request STJr/SRB2!906

Shader optimizations

See merge request STJr/SRB2!893

Funny shaders master merge

See merge request STJr/SRB2!892

shaders master merge and mipmap fix

See merge request !848

Flipfuncpointers (take two)

See merge request !864

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

See merge request !875

srb2:// server URLs

See merge request !858

Fix MD2 rollangle in reverse gravity

See merge request !818

OGL horizon lines

See merge request !861

ExecCfg fixes

See merge request !821

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

See merge request !868

Allow emblem hints (and radar) in record attack.

See merge request !820

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

See merge request !840

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

See merge request !796

Fix console backpic

See merge request !850

Turn FIXED_TO_FLOAT into a function

See merge request !869

Fix disconnected players being kicked for high ping

See merge request !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.