#15 using bt::textureResource in bbmail

[Joey Morris]

This patch replaces the texture-reading code in bbmail
with calls to bt::textureResource from libbt. This
gives bbmail better support for texture borders, and
fixes a rendering bug that appears to have happened
with "flat solid border" textures. In this bug, a white
border was drawn along the top and left sides of the
textures.

[Joey Morris]

bt::textureResource patch