Texture load functionality for integer types

Texture load functionality for integer types

- Added cases required to load integer type textures into
  swiftshader's internal formats.
- Cleaned up copy functions that simply perform a memcpy
  for a given number of bytes per row.
- Removed unused functionality for mini floats. Right now,
  all we need is the ability to import these formats, so
  there's currently no need to keep the export code.

Change-Id: I61500c41e668f885d3ec1e687b6f888c117221f2
Reviewed-on: https://swiftshader-review.googlesource.com/4140
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>