Add support for saving WebP images

[aaronfranke]

Godot already has code to save WebP images, but it's not exposed to the user. This PR refactors this code, adding and exposing APIs for saving WebP images.

The bulk of the saving code already existed in image_loader_webp.cpp, it has been moved to a new file webp_common.cpp, which is used by both the loader and the new file resource_saver_webp.cpp. This mimics the structure of the PNG code.

In addition to moving this code, this PR also fixes a bug @lyuma and @Macksaur discovered where Godot was adding an extra WEBP prefix that made the saved file not be a valid WebP file. This bug wasn't affecting us before because Godot only used this code to save stuff internally and then we stripped that prefix when loading internally, but it's technically a pre-existing bug that resulted in compressed textures being invalid WebPs and 4 bytes bigger than they needed to be.

The exposed methods are in the Image class. They mimic the API for saving PNG files, except that WebP has both lossy and lossless so there is an extra parameter for the quality setting. Passing in true for the lossy parameter will save lossy, and the quality can be set to a value between 0.0 and 1.0. Users can write image.save_webp("res://test.webp") to save a lossless WebP file, or image.save_webp("res://test.webp", true) to save a lossy WebP file at 75% quality, or image.save_webp("res://test.webp", true, 0.5) to save a lossy WebP file at 50% quality.

Special thanks to @lyuma and @Macksaur. My work on this was sponsored by The Mirror.

[Macksaur]

I like the proposed API with default parameters, +1. It makes for concise code and I'll never need to remember what true / false do.

[fire]

I'm approving this on a proposal standpoint, because webp is already one of godot accepted formats and is used widely for (Godot) lossless images.

It's open to technical review.

[akien-mga]

Looks good to me overall, still have to do a closer review.

[akien-mga]

Not needed anymore.

[akien-mga]

Discussed in PR review meeting, we agreed with @reduz that the API should be:

save_webp(String path, bool lossy = false, float lossy_quality = 0.75)

[akien-mga]

Looks good 👍

[akien-mga]

Thanks!