Compatibility: Improve gl texture format detection

[BlueCube3310]

Follow-up to #100856

This PR simplifies the flow of the _get_gl_image_and_format method as well as ensures that any decompressed format is passed directly into the output (previously, most decompressed formats were converted into RGBA8, including HDR ones).

This will ensure that the decompressed ASTC HDR textures will look the same as in Mobile and Forward+

Additionally, support for Image::FORMAT_RGB565 is now implemented, as before it was missing likely due to an oversight.

[fire]

This would be best reviewed by @clayjohn.

[Calinou]

Tested locally, it works as expected. However, I didn't encounter any issues when loading ASTC-compressed textures in Compatibility prior to this PR.

Code looks good to me.

Testing project: test_pr_101217.zip

I did some testing with various compressed formats and noticed an issue with ETC2 in particular. Also, notice how the ASTC memory usage is much higher in Forward+ than in Compatibility. (It actually takes a long time to load, which makes me think it's being decompressed on the CPU, but only in Forward+, not in Compatibility.)

Forward+

S3TC	ETC2	BPTC	ASTC

This error is printed every frame while previewing the ETC2 texture in the inspector:

ERROR: Attempting to use an uninitialized RID
ERROR: ./servers/rendering/renderer_rd/storage_rd/texture_storage.cpp:1411 - Parameter "tex" is null.

Compatibility

S3TC	ETC2	BPTC	ASTC

Unlike Forward+, I can use the ASTC-compressed HDRI as a sky texture, whereas it's just black in Forward+. I'm on a NVIDIA graphics card which lacks native ASTC support though.

[BlueCube3310]

Thank you for testing!

Also, notice how the ASTC memory usage is much higher in Forward+ than in Compatibility. (It actually takes a long time to load, which makes me think it's being decompressed on the CPU, but only in Forward+, not in Compatibility.)

Both are actually decompressed on CPU. The memory usage should be the same, it looks like Compatibility creates a copy of the source image before uploading it to GPU, whereas Forward+ operates on the source data

This error is printed every frame while previewing the ETC2 texture in the inspector:

ERROR: Attempting to use an uninitialized RID
ERROR: ./servers/rendering/renderer_rd/storage_rd/texture_storage.cpp:1411 - Parameter "tex" is null.

That is to be expected, it will be resolved by #100365

Unlike Forward+, I can use the ASTC-compressed HDRI as a sky texture, whereas it's just black in Forward+. I'm on a NVIDIA graphics card which lacks native ASTC support though.

That's surprising. it should work the same in Forward+
Edit: This happens because the engine cannot load .ctex files from the filesystem directly, but only via the imported pipeline, so it's unrelated to this.

[akien-mga]

Looks good to me.

[akien-mga]

Thanks!