ENH: Vulkan renderer compressed texture support

Addresses: #204 (closed), #205 (closed)

Includes various texture loading improvements:

• Reduced memory footprint with more compact formats for certain texture types
  • Single-channel texture types such as roughness, etc. now use only one 8 bits per channel as opposed to 32 originally
• DDS textures are supported (both compressed and uncompressed)
  • Support for mipmap loading is supported
• BCn texture compression types (see link below)
  • BC1-BC3 are supported
  • Due to what looks to be a bug with GLI, BC4 and BC5 are not supported at this time (https://github.com/g-truc/gli/issues/159)
  • BC6H and BC7 are not supported because I couldn't try out textures of these formats. However, they should be easy to add with the current refactoring.

http://www.reedbeta.com/blog/understanding-bcn-texture-compression-formats/