Software dropshadows

based on the accurate papersprite projection PR because the code is spaghetti-tangled together

TODO:

• use a dedicated graphic instead of stealing THOKA0
• possibly (MAYBE) remove the lightmap hack to make THOKA0 dark maybe later
• add configurable shadowscale property to objects and use it to determine what gets shadows - replace hardcoded list
• support the opened G-Ls someone else do this the ogl renderer scares me
• lots of testing

I cba to make them work in OGL so someone smarter than me can do that

sample very complicated wad for DSHADOW graphic: dshadow.wad