Small bug fixes to get the tests back to doing meaningful testing

Author: jholveck

src/mss/base.py

@@ -40,6 +40,7 @@ def __init__(
         self,
         /,
         *,
+        backend: str = "default",
         compression_level: int = 6,
         with_cursor: bool = False,
         # Linux only
@@ -51,6 +52,11 @@ def __init__(
         self.compression_level = compression_level
         self.with_cursor = with_cursor
         self._monitors: Monitors = []
+        # If there isn't a factory that removed the "backend" argument, make sure that it was set to "default".
+        # Factories that do backend-specific dispatch should remove that argument.
+        if backend != "default":
+            msg = 'The only valid backend on this platform is "default".'
+            raise ScreenShotError(msg)
 
     def __enter__(self) -> MSSBase:  # noqa:PYI034
         """For the cool call `with MSS() as mss:`."""

src/mss/linux/xcb.py

@@ -1179,6 +1179,10 @@ def xcb_setup_roots(r: XcbSetup) -> list[_Pointer]:
     return list_xcb(libxcb.xcb_setup_roots_iterator, libxcb.xcb_screen_next, r)
 
 
+def xcb_randr_get_screen_resources_crtcs(r: _Pointer) -> Array:
+    return array_xcb(randr.xcb_randr_get_screen_resources_crtcs, randr.xcb_randr_get_screen_resources_crtcs_length, r)
+
+
 def xcb_randr_get_screen_resources_current_crtcs(r: _Pointer) -> Array:
     return array_xcb(
         randr.xcb_randr_get_screen_resources_current_crtcs, randr.xcb_randr_get_screen_resources_current_crtcs_length, r