spearmint: Let game disable loading files for chat AI

Setting botlibvar "synfile", "rndfile", "matchfile", or "rchatfile"
to "" no longer tries to load the file.

World of Padman only uses matchfile.
spearmint: Fix GL2 skipping some shader lines

Lines after q3map_sun[Ext], q3gl2_sun, and q3gl2_tonemap were
skipped. Caused by specific keywords calling SkipRestOfLine,
because Spearmint calls SkipRestOfLineUntilBrace after done
parsing each line.

This broke ET's railgun sky shader.
Fix native trap_BotMutateGoalFuzzyLogic
Fix s_alDopplerSpeed default value
OpenGL2: Minor GLSL shader improvements.
OpenGL2: Add normalScale and parallaxDepth stage keywords and helper cvars.
spearmint: Allow disabling drawing sky for refdef

Used when sky is drawn separate from world, aka skybox portal.
OpenGL2: Set default normal/specular in RE_RegisterShaderFromImage
Merge pull request #57 from MAN-AT-ARMS/specularScale

OpenGL2: Set default normal/specular in RE_RegisterShaderFromImage
Merge remote-tracking branch 'ioquake3/master'

Conflicts:
	code/game/g_syscalls.c
Merge branch 'master' into sdl2