openfimg: Move compiler-related files to a subdirectory

Signed-off-by: Tomasz Figa <tomasz.figa@gmail.com>
openfimg: Compiler clean-up

Several clean-up related changes:
 - use rules-ng-ng to generate shader-specific definitions,
 - replace uses of bitfields with autogenerated helpers,
 - remove more dead code,
 - stylistic fixes.

Signed-off-by: Tomasz Figa <tomasz.figa@gmail.com>