Allow loading an image multiple times with different params

[slipher]

E.g. different nopicmip or imageMaxDimension.

Fixes Unvanquished/Unvanquished#2801

This could be useful if an image needs to be loaded with and without sRGB conversion (IF_ flags are part of the image params).

We have a handful of images in game assets that will be loaded multiple times:

image params mismatch for gfx/weapons/tracer/spark: 0x2 0 0/0 128 0 vs. 0x0 0 0/0 0 0
image params mismatch for models/buildables/arm/screen_s: 0x0 0 0/0 128 0 vs. 0x0 0 0/0 16 0
image params mismatch for models/players/human_male/base: 0x0 0 0/0 0 0 vs. 0x0 0 0/0 128 0

[illwieckz]

Great! Indeed we may want to render an image in the UI or to render the same image on a 3D surface, so many flags may differ (fitscreen, srgb…). One good example of that is an ATCS clone that replaced the end game firework sprite with the granger emoticon.

[illwieckz]

Maybe the _black, _red, _green_ and _blue images are exceptions too? At least for sRGB conversions 0 and 1 are always 0 and 1.

[slipher]

That would be possible, to set a boolean on image_t for built-in images that can ignore params: ones that have all pixels the same, and have color channels equal to 0 or 1. The practical benefits would be small though: _white, _black, and _flat are hard-coded into the shader parser (so it would never reach the image loader in most cases) and I don't think _red, _green, or _blue are used that often.

[illwieckz]

OK, this is good enough for a start.

I don't think _red, _green, or _blue are used that often.

I may one day modify the shared_colors materials to use them in engine. But that's not a big issue: those images are all very small.

[slipher]

Hmm actually I do need to implement something like that, since it is impossible to reload a builtin image from a file -- loading will just fail. But it needs to be for any builtin image, not just ones that are truly param-independent. Maybe I will just use the hack of checking if it begins with _.

EDIT: did this.

[illwieckz]

LGTM.