Fix #104514: GPencil merge down layer misses some frames

When merging two gpencil layers, if the destination layer had a keyframe
where the source layer did not, strokes of the previous keyframe
in source layer were lost in that frame.

This happened because the merge operator was looping through
frames of the source layer and appending strokes in the
corresponding destination layer, but never completing
other frames than the ones existing in the source layer.

This patch fixes it by first adding in source layer
all frames that are in destination layer.

@@ -1262,3 +1262,3 @@
           break;
         }
-        if ((gpf->next) && (gpf->next->framenum > cframe)) {
+        if (!(gpf->next) || (gpf->next->framenum > cframe)) {