Add 'alpha' field to objects

Adds a new object field that can be used to set an object's transparency without messing with frame flags. Range is 0 to FRACUNIT.

The FF_TRANS frame flags and the alpha field can be used simultaneously, with both affecting each other. For example, an object with FF_TRANS50 and an alpha of FRACUNIT/2 will be roughly 25% opaque.

Example Lua: MobjAlphaTesting.lua