tr_shader: do not load heatHaze map when heatHaze is disabled

[illwieckz]

Do not load heatHaze map when heatHaze is disabled.

This doesn't fix the root cause of #1736, but it workarounds it when heatHaze is disabled:

• htps://github.com/munmap_chunk(): invalid pointer when uploading heat haze maps on old low-end hardware can be worked around #1736

This is wanted in all cases because it's good to skip loading an uploading stuff we don't use anyway.

[slipher]

This spams parse errors and fails to really disable the stage:

Setting stage->active = false probably doesn't work there since it gets set to true 2 more times later - inside ParseStage, and after it returns as well. I ran into that while writing #1721 (which removes the redundant 2nd time setting it to true).

[illwieckz]

Hmm, right. I now disable the stage at the very end of the parsing (just before loading the image).

[slipher]

stage->active = false still doesn't work there since it is set again after ParseStage, as described in my previous comment. This results in messages such as Warn: Shader textures/plat23_custom/forcefield has a heatHazeMap stage with no image (ignored). But I think it would work if combined with my change from #1721 that deletes the final stage-active = true.

[illwieckz]

stage->active = false still doesn't work there since it is set again after ParseStage

Well, that is a bug unrelated to this.

[illwieckz]

I added a patch to not re-enable stages after parsing.

[slipher]

LGTM

[illwieckz]

LGTM

Thanks.