Wall lighting

Adds support for per-wall lighting, using two new UDMF fields for sidedefs:

• light: The amount to add or subtract from this side's sector brightness.
• lightabsolute: If set to true, this will make light an absolute value instead.

While per-texture lighting support is not added, this branch does add their relevant fields.

Test map: udmf-walllight.wad

Software	OpenGL