[Flutter GPU] Add Textures.

[bdero]

Going light on the docstrings so we can get to experimenting faster.

Textures! They can be written to by the host with Texture.overwrite and used as a ui.Image with Texture.asImage.

Right now we need to capture the "texture coordinate system" concept in order for Impeller to handle drawing textures in the right orientation on a Canvas. The user of the API may switch from host authoring to render target usage at any time. Similar to Unity, users targeting GLES will need to use this information to handle the texture flip when sampling from shaders -- this will be important to document comprehensively.

Flutter GPU textures being drawn with a CustomPainter:

Screen.Recording.2023-11-16.at.1.58.03.AM.mov

[jonahwilliams]

LGTM

Lets get this moving!

[chinmaygarde]

I figure we will keep tinkering on this further but I think we should stick to the underlying API more closely with allocators, descriptors, etc.. This PR is also a fascinating look into the kind of friction we will run into with FFI and something to think more deeply about.

[chinmaygarde]

Can this be more faithful to the the C++ API with the concept of descriptors and allocators obtaining texture handles?

[bdero]

Yeah I'm interested in hearing your thoughts around this. These breaks from Impeller are intentional, but indeed we'll keep tinkering and I'm super open to changing these things.

Will we ever need multiple device allocators? We ended up not using multiple for Entities, and most HALs seem to just tie allocations to the device. We did discuss the possibility of making contexts for multiple devices/having a device selection scheme in the future in the doc. There's a rough plan in case we need it, but those use cases are pretty eccentric.

The Flutter GPU doc removed a lot of descriptors from the beginning, opting for just using optional params in a lot of places instead. I think we should still have them for the more complicated config substructures like RenderPass attachments, but not sure we're gaining anything usability-wise by using them for texture and buffer creation?