4157: invalidate texture when unloading a texture. r=heinrich5991 a=ChillerDragon
Closes https://github.com/teeworlds/teeworlds/issues/1450
(cherry picked from commit 23a1b80f60)
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## Checklist
- [x] Tested the change ingame
- [ ] Provided screenshots if it is a visual change
- [ ] Tested in combination with possibly related configuration options
- [ ] Written a unit test if it works standalone, system.c especially
- [ ] Considered possible null pointers and out of bounds array indexing
- [ ] Changed no physics that affect existing maps
- [ ] Tested the change with [ASan+UBSan or valgrind's memcheck](https://github.com/ddnet/ddnet/#using-addresssanitizer--undefinedbehavioursanitizer-or-valgrinds-memcheck) (optional)
Co-authored-by: oy <Tom_Adams@web.de>
Co-authored-by: ChillerDragon <ChillerDragon@gmail.com>