Update Bob to handle compression DXT format (DEF-994)

[britzl]

Add methods to texc.cpp/.h that wraps PVRTexTools texture compression functionality. Expose methods to Java/Bob.
2015-02-04 14:26:51 (sven.andersson)
PVRTexTool does not support DXT compression on Mac or Linux.
Took a while to find, but to quote a rep from their forums:

??This is still the case I'm afraid - for legal reasons we had to disallow this as DXT compression/decompression should only be available on licensed platforms.??

There are alternative libs that do DXT compression, like:

• (LINK REMOVED)- (LINK REMOVED)
  2015-03-11 13:43:02 (sven.andersson)
  There is support for using DXT in the engine, Bob however does not support DXT compression yet.

Note that there is code to enable DXT usage in Bob that has been temporarily commented out until we have a texc that can compress DXT on all platforms.

Search for "DEF-994" to find all the relevant comments in the code.
2017-08-28 10:06:30 (andreas.tadic)
We will reside to use NVidia's TextureTool (sdk lib) which is free to use and includes the license for compressing DXT.

Not the least because nvtt is of good quality, which otherwise may vary substantially on different compressor libs / techs'.

Note: While implementing this, we should also look at what other features the texture tool has to bring to the table for Defold.