Text: allow for multiple SDF atlas textures

[lojjic]

The current implementation uses one texture, sized at 2048 wide. With a default glyph size of 64x64, in the 4 color channels, the 2048x2048 texture will hold 4096 glyphs.

That's enough for most cases, but in cases where there are many fonts/weights/styles in use, or character sets with large numbers of glyphs, or larger SDF sizes, or all of the above, it's conceivable the single texture won't be enough.

We should add the ability to overflow onto a second/third/etc. texture and update the shader to read from them.