Add wlb_output_set_funcs_with_size
Make EGL show up in header files only if EGL is already included
Add a generic mechanism for registering buffer types
Move the wlb_buffer_size_item structure inside compositor.c and properly destroy the buffer type list
Get rid of wlb_compositor_get_buffer_size and use the buffer_type directly
Use execl to avoid having to build an array with ill-defined constness