Remove termbox color buffer

Remove termbox color buffer

Removed the internal termbox background color buffer (which was only
used for the text background color when transparency was disabled) to
instead only use the clay color buffer.

Previously used two internal color buffers so that there wouldn't be
a need to convert from the clay to termbox representations for every
cell of text, but the performance difference will be negligible for
most TUI applications (which usually update on user input, or only a
few frames per second at most), so just using one buffer is cleaner