WINGs/wcolorpanel: gate magnifier via vtable callback dispatch

The magnifier is X11-backend-specific (screen capture). Instead of a
disguised is_x11 check, the view backend provides the callback directly:
- X11: .magnify_callback = W_ColorPanelMagnify (the magnifier impl)
- Wayland: .magnify_callback = NULL (button does nothing)

The button wiring uses the vtable value as the action callback.
No conditionals, no capability flags, pure vtable dispatch.

Also removes the screen_grab_image vtable slot that was a disguised check.

WINGs/WINGs/WINGsP.h.in

@@ -653,11 +653,15 @@ typedef struct W_ViewBackend {
                           int w, int h, const char *text);
     void  (*text_init)(void *vctx, struct W_View *view, void *stipple_gc_out);
     void  (*xor_fill_rect)(void *vctx, struct W_View *view, int x, int y, int w, int h);
+    WMAction *magnify_callback;
 } W_ViewBackend;
 
 extern const W_ViewBackend W_ViewBackendX11;
 extern const W_ViewBackend W_ViewBackendWL;
 
+/* Color panel magnifier (X11-only implementation) */
+void W_ColorPanelMagnify(WMWidget *w, void *data);
+
 
 /* ---[ color backend vtable ]-------------------------------------------- */
 

WINGs/wcolorpanel.c

@@ -277,7 +277,7 @@ static void closeWindowCallback(WMWidget * w, void *data);
 
 static Cursor magnifyGrabPointer(W_ColorPanel * panel);
 static WMPoint magnifyInitialize(W_ColorPanel * panel);
-static void magnifyPutCursor(WMWidget * w, void *data);
+
 static Pixmap magnifyCreatePixmap(WMColorPanel * panel);
 static void magnifyGetImageStored(W_ColorPanel * panel, int x1, int y1, int x2, int y2);
 static XImage *magnifyGetImage(WMScreen * scr, XImage * image, int x, int y, int w, int h);
@@ -405,7 +405,7 @@ static WMColorPanel *makeColorPanel(WMScreen * scrPtr, const char *name)
 	panel->magnifyBtn = WMCreateCustomButton(panel->win, WBBStateLightMask | WBBStateChangeMask);
 	WMResizeWidget(panel->magnifyBtn, 46, 36);
 	WMMoveWidget(panel->magnifyBtn, 6, 4);
-	WMSetButtonAction(panel->magnifyBtn, magnifyPutCursor, panel);
+	WMSetButtonAction(panel->magnifyBtn, scrPtr->viewBackend->magnify_callback, panel);
 	WMSetButtonImagePosition(panel->magnifyBtn, WIPImageOnly);
 	WMSetButtonImage(panel->magnifyBtn, scrPtr->magnifyIcon);
 
@@ -1693,10 +1693,11 @@ static WMPoint magnifyInitialize(W_ColorPanel * panel)
 	return point;
 }
 
-static void magnifyPutCursor(WMWidget * w, void *data)
+void W_ColorPanelMagnify(WMWidget * w, void *data)
 {
 	W_ColorPanel *panel = (W_ColorPanel *) (data);
 	W_Screen *scr = WMWidgetScreen(panel->win);
+
 	Cursor magCursor;
 	Pixmap pixmap;
 	WMEvent event;

WINGs/wrender_wl.c

@@ -276,4 +276,5 @@ const W_RenderBackend W_RenderBackendWL = {
     .fill_polygon = render_wl_fill_polygon,
     .fill_arc     = render_wl_fill_arc,
     .create_disabled_mask = render_wl_create_disabled_mask,
+
 };

WINGs/wrender_x11.c

@@ -172,4 +172,5 @@ const W_RenderBackend W_RenderBackendX11 = {
     .fill_polygon = render_x11_fill_polygon,
     .fill_arc     = render_x11_fill_arc,
     .create_disabled_mask = render_x11_create_disabled_mask,
+
 };

WINGs/wview_wl.c

@@ -622,4 +622,5 @@ const W_ViewBackend W_ViewBackendWL = {
 	.balloon_draw         = wl_balloon_draw,
 	.text_init            = wl_text_init,
 	.xor_fill_rect        = NULL,
+	.magnify_callback     = NULL,
 };

WINGs/wview_x11.c

@@ -581,4 +581,5 @@ const W_ViewBackend W_ViewBackendX11 = {
 	.balloon_draw         = x11_balloon_draw,
 	.text_init            = x11_text_init,
 	.xor_fill_rect        = x11_xor_fill_rect,
+	.magnify_callback     = W_ColorPanelMagnify,
 };