core.py: don't overwrite user's PIGLIT_SOURCE_DIR env var

When creating an Options object, use the environment's PIGLIT_SOURCE_DIR
value when available.

Fixes spec/ARB_ES3_compatibility/oes_compressed_etc2_texture-miptree test
failures in Cywin.

Also, remove the MESA_DEBUG=silent setting.  Piglit shouldn't care if
debug output is on/off (we stopped looking for Mesa error messages a
long time ago).

v2: simplify code with dict get() method and default value.

Reviewed-by: Dylan Baker <baker.dylan.c@gmail.com>
1 file changed