This holds a count byte followed by up to seven mask structures for the current room definition.

Used by the routines at setup_room and render_mask_buffer.

The first byte is a count, followed by <count> mask_t's, with the following format:

Type	Bytes	Name	Meaning
Byte	1	index	An index into mask_pointers (the RLE mask data pointer)
bounds_t	5	bounds	Isometric projected bounds of the mask. Used for culling.
tinypos_t	3	pos	If a character is behind this point then the mask is enabled. ("Behind" here means when the character's x-coord is greater and y-coord is greater-or-equal).