RConvertImage and RConvertImageMask now dispatch through function
pointers on RContext, set during context creation:
- RCreateContext sets x11_convert_image/x11_convert_image_mask
- RCreateContextWayland sets wl_convert_image/wl_convert_image_mask
The Wayland backend creates pixman_image_t from RImage RGBA data
instead of returning NULL pixmaps, fixing the SIGSEGV in the exit
dialog when compositing the application icon.
X11 code moved to convert_x11.c, Wayland code in convert_wl.c.
No dpy guards in the middle layer.
Levi Neely