check for null pointer when getting gaps

See: https://github.com/swaywm/sway/issues/4511

Adds a bool config option `primary_selection`, which explicitly
enables/disables the primary selection clipboard. Defaults to enabled.

This is implemented as a launch-only option which enables or disables the creation of the
`zwp_primary_selection_device_manager_v1` global.

Co-authored-by: Tilde Rose <t1lde@protonmail.com>

sway/desktop/render.c

@@ -285,6 +285,7 @@ void render_blur(struct fx_render_context *ctx, struct wlr_texture *texture,
 				.src_box = *src_box,
 				.dst_box = proj_box,
 				.transform = WL_OUTPUT_TRANSFORM_NORMAL,
+				.alpha = &deco_data.alpha,
 				.clip = &damage,
 				.filter_mode = WLR_SCALE_FILTER_BILINEAR,
 			},