PMX importer: Enable "Receive Transparent" for all materials.
PMX importer: Add debug prints for sphere textures to pmx loader.
PMX importer: Set "Receive" and "Receive Transparent" according to enabled_self_shadow.
PMX importer: Set "Cast Buffer Shadows" according to enabled_self_shadow_map.

When importing a pmd model, disable buffer shadows if material is alpha=0.98.
This change doesn't affect the ray shadows, so makes no sense in most cases.
PMX importer: Add option to specify if textures use MIP maps.

This avoids the issue that strange purple color may appear if a texture has 100% transparent areas.
However, disabling the MIP maps can cause the other problems, so this option defaults to ON.