Be more library friendly, by not aborting in library errors.
spice_common now includes a proper log handler that will abort by
default when reaching a warning.
SPICE_ABORT_LEVEL can be changed to modify run-time abort level.
SPICE_DEBUG_LEVEL can be changed to be more verbose. By default, only
log level more importants than WARNING.
Only memory-related functions are allowed to abort(), since they are
not recoverable errors in the library.
Since some spice C++ code is using code from common/, the C
functions need to be marked as such for the C++ compiler, otherwise
we'll get linkage issues.
Surface creation now specifies the exact format, not only the bit depth
of each surface which is used for rendering.
Additionally we now actually store the surfaces in that format, instead
of converting everything to 32bpp when drawing or e.g. handling palettes.
