Generate mip levels for DynamicTextMaps

This changeset introduced a new plBitmap flag `kAutoGenMipmap`, which
trickles through the pipeline and becomes `D3DUSAGE_AUTOGENMIPMAP` in
standard DirectX texture creation. This flag is applied to all
DyanmicTextMaps. The end result is that DynamicTextMaps become fuzzy at a
distance, rather than choppy.
Merge pull request #286 from Hoikas/mipmap-dtm

Automatic Mipmapping for DynamicTextMaps