Remove possible deadlock from imagemagick backend

This commit is tailored to be cherry-picked into a 2.10.3 release.

README.markdown

@@ -171,11 +171,12 @@ For some advanced uses of pqiv, take a look at these resouces:
 Changelog
 ---------
 
-pqiv 2.10.2
+pqiv 2.10.3
  * Fix output of `montage_mode_shift_y_rows()` in key bindings
  * Update the info text when the background pattern is cycled
  * Prevent potential crashes in poppler backend for rapid image movements
  * Fix processing of dangling symlinks in the file buffer
+ * Removed possible deadlock in ImageMagick wand backend
 
 pqiv 2.10
  * Enable cursor auto-hide by default

backends/wand.c

@@ -139,12 +139,17 @@ BOSNode *file_type_wand_alloc(load_images_state_t state, file_t *file) {/*{{{*/
 			new_file->private = g_slice_new0(file_private_data_wand_t);
 			((file_private_data_wand_t *)new_file->private)->page_number = n + 1;
 
+			// Temporarily give up lock to do this: Otherwise we might see a deadlock
+			// if another thread holding the file tree's lock is waiting for the wand
+			// lock for another operation.
+			G_UNLOCK(magick_wand_global_lock);
 			if(n == 0) {
 				first_node = load_images_handle_parameter_add_file(state, new_file);
 			}
 			else {
 				load_images_handle_parameter_add_file(state, new_file);
 			}
+			G_LOCK(magick_wand_global_lock);
 		}
 
 		if(first_node) {