Add dofile() to Lua

See merge request STJr/SRB2!969

Custom skincolor bugfixes

See merge request STJr/SRB2!971

Improvements to the in-game file downloader

See merge request STJr/SRB2!976

# Conflicts:
#   src/d_clisrv.c

Add a Snake minigame to the file transfer screen

See merge request STJr/SRB2!903

Strike null-pointer-arithmetic error in BUFFER_OFFSET

See merge request STJr/SRB2!989

Fix OPENMPT deprecation error, openmpt_module_ctl_set

See merge request STJr/SRB2!990

Fix desynch when a player spawns

See merge request STJr/SRB2!977

Dashmode fixes

See merge request STJr/SRB2!909

Fix mouse being grabbed in continue screen

See merge request STJr/SRB2!983

Don't cut momentum if landing in a roll

See merge request STJr/SRB2!972

Cppcheck fixes

See merge request STJr/SRB2!967

Allow transparent parts in translucent FOF and PolyObject planes

See merge request STJr/SRB2!978

Sync turnobjs for polyrotate_t thinkers in netgames

See merge request STJr/SRB2!979

sdl/i_main.c: while we're here, split all the logging initialisation code into a separate function, to make main function more clean

(though that said we don't officially care about the win32 folder files anymore)

V_Init: count to NUMSCREENS, not NUMSCREENS+1 (though I can't help thinking this one might have been deliberate somehow?)

Polyobj_rotate and related: change "origin" to vector2_t since all it needs is x/y and not any of the new stuff for vertex_t

This way, if for some reason the ghost has no skin, nothing should spawn at all

Effect 6 flag allows cutting of cyan pixels on FOF and PolyObject planes (even if they're translucent)

# Conflicts:
#   src/lua_script.c
#   src/lua_script.h