
TEXTURE COORDINATE SYSTEM
=========================

'u' means horizontal offset / position
'v' means vertical offset / position (Z axis)

bottom texture: bu, bv fields in brush sides
top    texture: tu, tv fields in brush sides
(usually placed in every side, but if only in a single side
then the texture is axis aligned and 1:1 scale)

side texture: u1 and/or u2 in brush sides
              v1 and/or v2 in brush sides
(if only one of v1/v2 is present, other is computed from brush height)
(u2 can be absent, but u2 without u1 is not supported)
(can put v1 in t coord, v2 in b coords, for default value of all sides)

new coord kind: zv = height, used with v1 or v2 to vertically anchor
all the side textures (without any v1 or v2) to a certain height.

x_offset is a synonym for u1
y_offset is a synonym for v1

DOOM and other games don't support scaling.  When both u1 and u2, or
v1 and v2 are present on brush sides, only u1 or v1 are used.
Similarly any floor/ceiling texture coordinates are ignored.

The 'peg' flag is only for DOOM-based engines, causing the texture to
move with the sector's floor or ceiling -- needed for doors and lifts,
and useful in some other places (like alignment on window lowers).
This flag will change how texturing is applied (in the DOOM engine)
but has no effect on how the final y_offset is computed.

