WM: support checking windowing capabilities

Historically checks for windowing capabilities used platform
pre-processor checks however that doesn't work when Blender is built
with both X11 & Wayland.

Add a capabilities flag which can be used to check which functionality
is supported. This has the advantage of being more descriptive/readable.

source/blender/windowmanager/WM_api.h

@@ -126,6 +126,15 @@ void WM_init_opengl(void);
  */
 const char *WM_ghost_backend(void);
 
+typedef enum eWM_CapabilitiesFlag {
+  /** Ability to warp the cursor (set it's location). */
+  WM_CAPABILITY_CURSOR_WARP = (1 << 0),
+  /** Ability to access window positions & move them. */
+  WM_CAPABILITY_WINDOW_POSITION = (1 << 1),
+} eWM_CapabilitiesFlag;
+
+eWM_CapabilitiesFlag WM_capabilities_flag(void);
+
 void WM_check(struct bContext *C);
 void WM_reinit_gizmomap_all(struct Main *bmain);
 

source/blender/windowmanager/intern/wm_window.c

@@ -1646,6 +1646,23 @@ GHOST_TDrawingContextType wm_ghost_drawing_context_type(const eGPUBackendType gp
   return GHOST_kDrawingContextTypeNone;
 }
 
+eWM_CapabilitiesFlag WM_capabilities_flag(void)
+{
+  static eWM_CapabilitiesFlag flag = -1;
+  if (flag != -1) {
+    return flag;
+  }
+
+  flag = 0;
+  if (GHOST_SupportsCursorWarp()) {
+    flag |= WM_CAPABILITY_CURSOR_WARP;
+  }
+  if (GHOST_SupportsWindowPosition()) {
+    flag |= WM_CAPABILITY_WINDOW_POSITION;
+  }
+  return flag;
+}
+
 /** \} */
 
 /* -------------------------------------------------------------------- */