For Comments: cache2 fixed sizes

Now that the thumbnail mip levels are all in the same cache we can have more mip levels spanning more resolutions so as to seamlessly allow switching between 720p/1080p/4K screens. This patch does two things:

  * Add more thumbnail mip levels (DT_MIPMAP_4 and DT_MIPMAP_5)
  * Set fixed sizes for the levels that try to map to commonly used resolutions

Adding more mip levels is now just a matter of adding them to the enum in mipmap_cache.h and the size to the table in mipmap_cache.c.