Added initial image support. Renders using characters with several
options (ascii, foreground, background, unicode, etc).
Note that each image object should only be used once (or at least with
only one size), otherwise the cached results will conflict for each
size and rendering will take longer and reduce responsiveness.
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