VSE: speedup timeline drawing

The strips and retiming items were drawn one at a time, switching
between GPU shaders, geometry topologies and using tiny immediate
mode batches (often two triangles) for everything.

Optimize that by going way larger GPU batches, like:
- Draw all strip backgrounds at once,
- Draw all strip handles at once,
- Draw all strip outlines at once,
- Draw all retiming continuity sections at once,
- Draw all retiming keyframes at once.

There's no efficient way to draw separate quads in GPU IMM API, so
instead this adds SeqQuadsBatch utility that batches up to 1024
quads for drawing, using indexed vertices.

In Sprite Fright Edit data set, with whole timeline visible (2702
strips), repainting the timeline UI with audio waveforms on takes:

Windows (Ryzen 5950X, RTX 3080Ti): 46ms -> 23ms. Of the time still
left, 14ms is drawing sound waveforms. Remaining cost is other overheads
not directly related to actual rendering (some squared complexity
selection queries, f-curve existence lookups etc.).

@@ -0,0 +101,4 @@
+    draw();
+  }
+  if (quads_num == 0) {
+    /* Don't actually need this, but it's the only way to clear GPU_VERTBUF_DATA_UPLOADED flag. */

@@ -0,0 +105,4 @@
+    GPUVertBufRaw attr_data;
+    GPU_vertbuf_attr_get_raw_data(vbo_quads, 0, &attr_data);
+
+    verts_quads = (ColorVertex *)GPU_vertbuf_get_data(vbo_quads);

@@ -313,6 +303,12 @@ static void retime_key_draw(const bContext *C, const Sequence *seq, const SeqRet
   CLAMP(key_position, left_pos_min, right_pos_max);
   const float alpha = SEQ_retiming_data_is_editable(seq) ? 1.0f : 0.3f;
 
+  if (*r_point_counter >= MAX_KEYS_IN_BATCH) {

@@ -414,2 +415,4 @@
   }
 
+  /* Get the selection here once, for faster "is key selected?" lookups later.
+     This is similar to SEQ_retiming_selection_get, except we want only a set. */

@@ -637,3 +518,1 @@
-    float y_mid = (strip_ctx->bottom + strip_ctx->strip_content_top) / 2.0f;
-    /* The length from the middle of the strip to the top/bottom. */
-    float y_scale = (strip_ctx->strip_content_top - strip_ctx->bottom) / 2.0f;
+    /* Do not draw the bars below 2px height. */

@@ -333,0 +315,4 @@
+static void draw_continuity(const bContext *C,
+                            const Sequence *seq,
+                            const SeqRetimingKey *key,
+                            const blender::Set<const SeqRetimingKey *> *selection,

@@ -432,0 +460,4 @@
+    }
+
+    /* If batch is full, start a new one (fake keys add 2 points max). */
+    if (point_counter + 2 > MAX_KEYS_IN_BATCH) {