Fixes #5246

This adds a new special overflow atlas page of the maximum device texture size when large glyphs are needed. This is inherently slower to upload to the GPU, but it's a one time cost per glyph. This is how it looks after writing a series of - and =:

Notes:

• There isn't any handling for when this overflows that texture as it will likely never happen in practice.
• This should also avoid the merge logic as the texture size is already the largest supported by the GPU.
• There are many ways we could improve this, like handling ligatures on a per cell level and then de-duplicating identical glyphs. This would need a fairly large refactor and we'd probably want to simplify the texture atlas merging if we went this route.