vtfpp: add support for Dreamcast PowerVR textures

[erysdren]

not done yet.

• check if i need to add any new formats to the vtfpp::ImageFormat enum (probably will, for VQ compressed textures)
• validate that texture dimensions are power-of-two
• get some example files from Dreamcast Half-Life or Quake 3
• make sure detwiddling is implemented for RGBA conversions

questions:

• do we need writing code?
  • would have to have VQ compression and twiddling support, as well as palletization