Remove special case for SP homing missiles

The special case let robot homing missiles track players without a line
of sight and prevented player homing missiles from tracking robots out
of the player's sight.
Move interpreter OP_ constants to static const
Remove unused !FADE_FLATPOLY code
Move interpreter g3s_lrgb onto stack

MAX_POINTS_PER_POLY is 25, so storing lrgb on the stack fits easily and
saves memory allocation/deallocation.
Propagate NULL arguments to g3_check_and_draw_poly
Remove write-only variable morph_uvls
Change isyou to your_index and promote it out a layer

There is only ever one record for which isyou is true.
Only set canvas color if it is used
Move point_list onto interpreter stack
Use array<> for textures_list