GPv3: Dissolve Operator.

The problem was that when dissolving between and a end point was selected the first/last range would not have had the first/last point in it. and so it would not dissolve some of the regions.

@@ -561,0 +563,4 @@
+ * \{ */
+
+enum DissolveMode {
+  /* dissolve all selected points */

@@ -561,0 +630,4 @@
+                    curve_selection, false);
+
+                if (deselection_ranges.size() != 0) {
+                  IndexRange first_range = deselection_ranges.first().shift(points.first());

@@ -561,0 +635,4 @@
+
+                  /* Because we are going to invert, fill with `!false` so that it does not get
+                   * dissolved.*/
+                  points_to_dissolve.as_mutable_span().slice(last_range).fill(!false);

@@ -561,0 +613,4 @@
+        /* Both `between` and `unselect` have the unselected point being dissolved.*/
+        if (mode == DISSOLVE_BETWEEN || mode == DISSOLVE_UNSELECT) {
+          threading::parallel_for(curves.curves_range(), 128, [&](const IndexRange range) {
+            for (const int curve_i : range) {

@@ -561,0 +620,4 @@
+              if (!curve_selection.contains(true)) {
+                /* Because we are going to invert, fill with true so that it does not get
+                 * dissolved.*/
+                points_to_dissolve.as_mutable_span().slice(points).fill(true);

@@ -561,0 +648,4 @@
+        IndexMaskMemory memory;
+        IndexMask points_to_remove = IndexMask::from_bools(points_to_dissolve, memory);
+
+        if (!points_to_remove.is_empty()) {

@@ -561,0 +674,4 @@
+  /* Callbacks. */
+  ot->invoke = WM_menu_invoke;
+  ot->exec = grease_pencil_dissolve_exec;
+  ot->poll = editable_grease_pencil_poll;

@@ -561,0 +595,4 @@
+  }
+  /* Both `between` and `unselect` have the unselected point being the ones dissolved so we need
+   * to invert.*/
+  else if (mode == DISSOLVE_BETWEEN || mode == DISSOLVE_UNSELECT) {

@@ -561,0 +607,4 @@
+        const Span<bool> curve_selection = points_to_dissolve.as_span().slice(points);
+        /* The unselected curves should not be dissolved.*/
+        if (!curve_selection.contains(true)) {
+          points_to_keep.slice(points).fill(true);

@@ -561,0 +611,4 @@
+        }
+        /* `between` is just `unselect` but with the first and last segments not geting
+         * dissolved.*/
+        else if (mode == DISSOLVE_BETWEEN) {

@@ -561,0 +599,4 @@
+  }
+
+  /* Both `between` and `unselect` have the unselected point being the ones dissolved so we need
+   * to invert.*/

@@ -561,0 +603,4 @@
+  BLI_assert(ELEM(mode, DissolveMode::BETWEEN, DissolveMode::UNSELECT));
+
+  const OffsetIndices<int> points_by_curve = curves.points_by_curve();
+  /* Because we are going to invert, these become the points to keep.*/