Allow default, min, max atlas size to be specified

[grovesNL]

No description provided.

[genusistimelord]

This should also include limitations based on the System. You can get these lower and higher limitations from WGPU. You do not want to allow sizes that are below or above the systems sizes max/min. Generally since it is not super important end user side it would be best to preset this to the max texture size that the System would support. As you will need all the space you can get for Glyphs. Unicode along without storing different sizes of glyph's can kill the texture pretty fast.

[Nuckal777]

I have played around with dynamically growing the texture atlas. There is two ways to do this:

1. Allocate a larger texture, drop glyph_cache, packer and texture_pending and rerasterize/reupload every currently required glyph (run prepare again).
2. Allocate a larger texture, do a texture to texture copy using wgpu::CommandEncoder and extend texture_pending and packer. etageres Allocators don't support growing, though. Alternatively, this crate provides a growable atlas.

If you can tell me, which approach would be preferred, I am happy to drop by a PR.

[genusistimelord]

I find its generally better to have a Texture array for each of the textures starting out at their max size of what is supported by hardware. Then using guillotiere to tell me if we need another layer or not. In my own project this is how i do it as this avoids resizing and allows dynamically growing within needed space. Also prevents you from over drafting the texture sizes on devices that don't support it.

[grovesNL]

A growable atlas is being implemented in #34