Support for packed sprite format and more

- New packed sprite format which stores several frames on one texture
(usually only one per sprite). Packed sprite uses IDSP header and
loader, provided as a sprite versions 128 (8-bit packed sprite with
internal 32-bit colormaps) and 160 (32-bit packed sprite). OmniTool util
can be used to produce packed sprites.
- DDS loader: now can load pre-stored avgcolor directly from unused DDS
header (RwgTex already supports avgcolor storage in DDS files). New cvar
r_texture_dds_load_avgcolormode controls the behavoid (defaults to old
one - decoding DDS contents to get avgcolor).