Add clipmidtex property to lines and sides

General

Adds clipmidtex from ZDoom.

If true, this side's mid textures are clipped to floor and ceiling. Default is to let them "bleed" through floor and ceiling planes. Note that OpenGL renderers will clip mid textures regardless.

Source: https://zdoom.org/wiki/Sidedef

If clipmidtex is used on a polyobject side, this clips the polyobject's textures against its heights, rather than extending past them.

Example maps

clipmidtex-test.wad
clipmidtex-test2.wad

UDMF

Changes to the UDMF specification.

Added to sides

Field	Description
clipmidtex	Side's mid textures are clipped to floor and ceiling.

Added to lines

Field	Description
clipmidtex	Side's mid textures are clipped to floor and ceiling.

Lua

Changes to the Lua API.

Added to sides

Field	Description
clipmidtex	Whether the middle texture is clipped to the ceiling and the floor of the sector.