GPv3: Copy/Paste Keyframes

This commit implements Copy and Past Keyframes for GPv3. The operation
is available in Animation workspace. The copy operator copies the
selected GPv3 Keyframes and the paste operator paste the Keyframe from
the clipboard.

**Technical notes**
`grease_pencil_copy_keyframes()` iterates through every selected frame
for every channel/layer, then stores frame_number and layer name in a
vector of `BufferItem`. Please note that layer can be moved so storing
layer pointer is not viable.
`grease_pencil_paste_keyframes()` iterates through all `BufferItem` and
uses `GreasePencil::insert_duplicate_frame()` to duplicate the keyframe
and drawing. Please note that there's a issue with the function that may
lead to crash, for which I opened #114352.

@@ -382,0 +408,4 @@
+  Object *object = ac->obact;
+
+  /* filter data */
+  filter = (ANIMFILTER_DATA_VISIBLE | ANIMFILTER_LIST_VISIBLE | ANIMFILTER_NODUPLIS);

@@ -382,0 +421,4 @@
+
+    GreasePencil *grease_pencil = reinterpret_cast<GreasePencil *>(ale->id);
+    blender::Span<const Layer *> layers = grease_pencil->layers();
+    for (const Layer *layer : layers) {

@@ -382,0 +484,4 @@
+      break;
+  }
+
+  filter = (ANIMFILTER_DATA_VISIBLE | ANIMFILTER_LIST_VISIBLE | ANIMFILTER_NODUPLIS);

@@ -381,1 +382,4 @@
 
+/* Datatype for use in copy/paste buffer */
+struct BufferItem {
+  Vector<int> frame_numbers;

@@ -382,0 +407,4 @@
+  Scene *scene = ac->scene;
+  Object *object = ac->obact;
+
+  /* filter data */

@@ -382,0 +498,4 @@
+    blender::Span<Layer *> layers = grease_pencil->layers_for_write();
+    for (const BufferItem buffer : copy_buffer) {
+      for (Layer *layer : layers) {
+        if (layer->name() == buffer.layer_name) {

@@ -382,0 +505,4 @@
+          }
+        }
+      }
+    }

@@ -434,0 +556,4 @@
+    blender::Span<Layer *> layers = grease_pencil->layers_for_write();
+    bool change = false;
+    for (const BufferItem buffer : copy_buffer) {
+      for (Layer *layer : layers) {

@@ -434,0 +581,4 @@
+          /* remove all keys */
+          Vector<int> frames_to_remove;
+          for (auto frame_number : layer.frames().keys()) {
+            frames_to_remove.append(frame_number);

@@ -434,0 +564,4 @@
+    GreasePencil *grease_pencil = reinterpret_cast<GreasePencil *>(ale->id);
+    bool change = false;
+
+    for (auto [layer_name, layer_buffer] : copy_buffer.items()) {

@@ -434,0 +616,4 @@
+        }
+      }
+      for (auto drawing_buffer : layer_buffer.drawing_buffers) {
+        int target_frame_number = drawing_buffer.frame_number + offset;

@@ -434,0 +620,4 @@
+      if (layer->frames().contains(target_frame_number)) {
+        layer->remove_frame(target_frame_number);
+      }
+      layer->add_frame(target_frame_number, grease_pencil->drawings().size(), 0);

@@ -434,0 +447,4 @@
+
+/* Globals for copy/paste data (like for other copy/paste buffers). */
+static Map<std::string, LayerBufferItem> copy_buffer;
+static int copy_buffer_first_frame = INT_MAX;

@@ -434,0 +565,4 @@
+                           ac->data,
+                           eAnimCont_Types(ac->datatype)) == 0)
+  {
+    ANIM_animdata_filter(

@@ -434,0 +582,4 @@
+      continue;
+    }
+    LayerBufferItem layer_buffer = from_single_channel ? *copy_buffer.values() :
+                                                         copy_buffer.lookup(layer_name);

@@ -10,6 +10,7 @@
 #include <cmath>
 #include <cstdlib>
 #include <cstring>
+#include <stdio.h>

@@ -434,0 +445,4 @@
+  blender::bke::greasepencil::FramesMapKey last_frame;
+};
+
+/* Globals for copy/paste data (like for other copy/paste buffers). */

@@ -434,0 +537,4 @@
+  int offset = 0;
+
+  /* Methods of offset (eKeyPasteOffset). */
+  switch (offset_mode) {

@@ -434,0 +621,4 @@
+        break;
+      }
+    }
+    for (auto drawing_buffer : layer_buffer.drawing_buffers) {

@@ -434,0 +624,4 @@
+    for (auto drawing_buffer : layer_buffer.drawing_buffers) {
+      const int target_frame_number = drawing_buffer.frame_number + offset;
+      if (layer->frames().contains(target_frame_number)) {
+        layer->remove_frame(target_frame_number);

@@ -434,0 +446,4 @@
+};
+
+struct Clipboard {
+  Map<std::string, LayerBufferItem> copy_buffer;

@@ -434,0 +569,4 @@
+  const int offset = calculate_offset(offset_mode, ac->scene->r.cfra, clipboard);
+
+  const int filter = (ANIMFILTER_DATA_VISIBLE | ANIMFILTER_LIST_VISIBLE | ANIMFILTER_NODUPLIS |
+                      ANIMFILTER_FOREDIT | ANIMFILTER_SEL);