libgui|GLBuffer|GLProgram: Added more vertex attributes for texture bounds
libgui|Fixed|Atlas: Applying a border when no background store is in use

Todo: Not a finalized implementation, but it'll do for now...
Refactor|Cleanup: Model definition identifiers
libgui: Added HeightMap

This is a class I've written earlier for working with height maps.
Allows determining height at any point, and producing a normal map.
libgui|ModelDrawable: Normal and height maps

The vertex data handled my ModelDrawable now contains information for
specifying the normal map for the geometry. This can come from the
model file (if specified), or it can use a common default normal map
(e.g., simple +Z vector). Also, one can manually specify a custom file
as the normal map.
Model Renderer: Working on normal mapping

The generic model shader now supports normal mapping. The test model
uses a height map image that gets automatically converted to a normal
map by ModelDrawable.

Todo: The definition syntax for specifying custom textures in materials
of a model needs revising.