lines, or on changed chars if we have a mask

a more natural (x,y,w,h) 4-tuple to handle.

inner implementation still only handles one dirty rectangle, but this way
we can prepare supporting applictions for the future.

split is rendered moot by the plugin system: when enabled, binaries do
not link directly with libX11 or libGL. I hope this is a step towards
more consisteny and clarity.

<cucul.h> will provide the C99 types, even if libcaca has been installed.
  * Rename what's left of "common.h" to "stubs.h".
  * Remove all references to erroneous <inttypes.h> from source files.

* Added a first, simple test program.