core: separate work with the x extensions

[absolutelynothelix]

1. a todo is addressed:

   picom/src/x.c

   Line 67 in 1648673

   // TODO(yshui) separate error code out from session_t

2. unused information about the x extensions (base event and error numbers, major opcode) is not stored anymore;
3. information about the x extensions (presence, base event and error numbers) is now stored in a single, separate place - a structure called x_extensions that lives in the x_connection structure, so the session structure is less polluted. it's ordered and clean so it's clear how to add and/or remove the x extensions;
4. the x extensions are now initialized in a single, separate place - a function called x_init_extensions that is called right after the x_init_connection function, so the session_init function is less polluted. it's ordered and clean so it's clear how to add and/or remove the x extensions. also it separates initialization from work (only initialization happens there, all the work left in the session_init function), has comments (when a particular x extension initialization happens, why we require particular versions of particular x extensions and why we sometimes negotiate versions of x extensions just to discard this information) and has a better error handling (when a required x extension is missing or something goes wrong it at least tries to free what's already allocated instead of immediately dying using exit(1) and leaking a bunch of stuff);
5. --diagnostics reports presence of all the optional x extensions in the same way now;
6. the win_update_bounding_shape function doesn't require shape_exists that indicates the x shape extension presence, it determines it itself using c;
7. the x_error_code_to_string function doesn't use ps_g anymore, it uses c instead;
8. the x extensions used by picomling are now properly initialized and a picomling won't born if one of them is missing.

[absolutelynothelix]

@yshui, let's resolve a few concerns before merging this.