Nobody cares about BeOS in 2022, and it cannot be built with Meson anyway.
Removing the BeOS backend also allows us to drop the dependency on a C++
compiler and simplify the Autotools build considerably by removing all
the extra CXX targets.
This is consistent with the naming of most cairo types/functions
(example: cairo_foo_surface_*).
The substitution in the code has been performed using:
sed -i 's/cairo_pattern_mesh_/cairo_mesh_pattern_/' <files>
Clarify that cairo_pattern_create_rgb() and cairo_pattern_create_rgba()
should be be overloaded as a single constructor for SolidPattern, but
instead should be static methods.
