mesa: Add decoding functions for GL_COMPRESSED_SIGNED_R11_EAC

mesa: Add decoding functions for GL_COMPRESSED_SIGNED_R11_EAC

Data in GL_COMPRESSED_SIGNED_R11_EAC format is decoded and stored in
MESA_FORMAT_SIGNED_R16.

v2:
16 bit signed data is converted to 16 bit unsigned data by
adding 2 ^ 15 and stored in an unsigned texture format.

v3:
1. Handle a corner case when base code word value is -128. As per
OpenGL ES 3.0 specification -128 is not an allowed value and should
be truncated to -127.
2. Converting a decoded 16 bit signed data to 16 bit unsigned data by
adding 2 ^ 15 gives us an output which matches the decompressed image
(.ppm) generated by ericsson's etcpack tool. ericsson is also doing this
conversion in their tool because .ppm image files don't support signed
data. But gles 3.0 specification doesn't suggest this conversion. We
need to keep the decoded data in signed format. Both signed format
tests in gles3 conformance pass with these changes.

Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Tested-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Brian Paul <brianp@vmware.com>