Support for precompressed lightmaps, optimizations for q3bsp loader

- R_SkinFrame_LoadExternal: added option for basetex-only skinframe
(reduce load times by reducing file search calls).
- new cvar mod_q3bsp_lightmapskinframes which forces skinframes path for
lightmaps (so precompressed lightmaps can be loaded from dds/).
Lightmap skinframes do not support merging, so it is recommended to use
it with maps with one large lightmap.
- Covered case when map have only one external lightmap, for that case
merging are not performed. This often saves video ram (as merged
lightmap can be bigger than existing) and some CPU time.
- Added support for YCoCg Scaled texture compression for lightmaps.
YCoCg compression described here:
http://www.nvidia.com/object/real-time-ycocg-dxt-compression.html . This
implementation have three differences: precompression (not realtime),
full range scale factor and Gamma 2.0 textures (which have better
precision for dark zones). This introduces two new types of DDS files to
be loaded: 'YCG2' for YCoCg Scaled, 'YCG4' for YCoCg Scaled Gamma 2.0.
DDS files are identified as DXT5, while YCG2/YCG4 identifiers are placed
in byte 44. This is way Gimp DDS writes and saves YCoCg files (although
it does not support YCG4, which can be produced by RwgTex tool).
- QuakeWorld checksum calculation for BSP files is now optional (
mod_bsp_qwchecksum ) as singleplayer games such as Blood Omnicode do not
connect to QW servers and disabling QW checksum saves some CPU time
(making map loads faster).
- Some Q3map2 bug fixing routines are not optional (
mod_q3bsp_fixq3map2bugs  ), since modern version of q3map2 fixes this
bugs. This saves some CPU time and makes map load faster.
- New shader keyword "dpnobih" that disables BIH tree generation for
surface. Used as an optimization for map load times (since some geometry
such as grass never need BIH anyway).
- Hush a warning "unknown surfaceparm" for surfaceparm monsterclip.