(start) some style syntax revisions after a discussion on the mailing

(start) some style syntax revisions after a discussion on the mailing
list:

<name>.backgroundColor:	background color for solids
<name>.color1: 		initial color for gradients and interlaced solids
<name>.color2:	 	target color for gradients and interlaced solids

this also entails a slight change in the bt::Texture API... instead of
bt::Texture::color() and ::colorTo(), we have ::color1() and
::color2().  Note that there is no bt::Texture::backgroundColor(); use
::color1() instead.