A new ETC2 decoder was added to SwiftShader, based on the
OpenGL ETC2 specification. The decoder is fully standalone
and does not rely on any outside code (there are no files
included in the header and the source file only include the
header file, so it can easily be ported to any other project).

Things to note:
- In Surface.cpp, signed ETC2 images are decoded to full 32FP
  images, because of the lack of support for signed 8 bit R
  and RG internal formats. This should be fixed as soon as
  these formats are made available.
- sRGB conversion is not performed within the decoder, so it has
  been added as a loop inside Surface::decodeETC2 after the ETC2
  decoding is performed. This is to make sure that there is no
  loss of precision, should we choose to do the conversion to a
  higher bit precision format. The loop is fairly straightforward
  and does the conversion in place, so the impact on performance
  compared to doing the sRGB conversion in the decoder should be
  minimal.

Change-Id: I3a1af623353344bf35818ba9c9f4cf349b587e2f
Reviewed-on: https://swiftshader-review.googlesource.com/3960Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>