pico: detect intentional material name and do not replace them with texture

pico: detect intentional material name and do not replace them with texture

Detect intentional material name to not replace it with texture name.

Reimplement commits by Garux:
https://github.com/Garux/netradiant-custom/commit/1bd3e7ae186b55fb61e3738d2493432c0b1f5a7b
https://github.com/Garux/netradiant-custom/commit/ea21eee2254fb2e667732d8f1b0f83c439a89bfa

This attempts to restore proper material behaviour when the mapper knows what he is doing,
also called Julius' case or correct case because Julius is always correct™
while keeping the fallback for other situations, also called newbie's case
which may be compatible with third-party tools not following Quake 3 conventions.

See: https://gitlab.com/xonotic/netradiant/-/merge_requests/179#note_777324051

<3 @julius__ @Garux