ui: handle texture size limit

[deltabeard]

There are currently no checks in place to make sure that created textures are within the texture size limit. Someone using a graphics card with a limit of 64x64 texture will be unable to use this toolkit, for example.

To remedy this, the texture size limit should be recorded on initialisation, and textures should:

1. Be resized to fit within the texture size limits.
2. Create multiple textures for a single image if required.
3. Or both.

Related to #31.