Added non-functional skeleton code for XPM parser using GNU Flex/Bison
Fixed some compiler warnings in img_priv.c and img_string.c.

Additionally, I fixed a major bug in img_vec_insert.
The dimensions of len did not match up with the dimensions of
idx in the third parameter to memmove.  As a result, img_vec_insert
was not working when vec->elem_sz > 1.  This has been resolved.
Got the XPM loader working.  There's still some error-checking/input validation and general
cleanup which needs to be done before I push this to github, but it works.
Added in input validation, error handling to xpm parsing
fix a buffer-overflow vulnerability in the xbm lexer
Just realised there are a few features from XPM that I didn't know about.
For now, I'm ignoring symbolic colors and 'None' by replacing them with grey.
I also need to handle color types that are not 'c'; for now it still rejects
those as invalid.
Added XPM test case