Tweaks to TextureAtlasBuilder.finish()

[4-rodrigo-salazar]

This does a couple things:

• We don't enforce that initial and max sizes are powers of 2, but when finding a size that fits we multiple by 2 in every iteration. This logic is updated so that we at least try the max_size value that the user provided, before this change it would just jump over it and exit early without trying the largest size the user specified. This means that previously we could return NotEnoughSpace even though the max size provided is actually greater than the space you need.
• Minor: Move the Texture::new_fill() call so that we only call it if pack_rects returns successfully.
• Minor: Remove unnecessary "rect_placements = None" on line 89.

Tested by tweaking our examples/2d/texture_atlas.rs with various initial/max sizes.

[cart]

Good improvements all around. Thanks!