gsk: Don't overflow vbo_count

We use 16 bits to store vbo_count, so we can't create
batches that have more than 65535 vertices. Pay attention
to that limit when merging batches.

gsk/ngl/gsknglcommandqueue.c

@@ -617,6 +617,7 @@ gsk_ngl_command_queue_end_draw (GskNglCommandQueue *self)
       last_batch->any.viewport.height == batch->any.viewport.height &&
       last_batch->draw.framebuffer == batch->draw.framebuffer &&
       last_batch->draw.vbo_offset + last_batch->draw.vbo_count == batch->draw.vbo_offset &&
+      last_batch->draw.vbo_count + batch->draw.vbo_count < G_MAXINT16 &&
       snapshots_equal (self, last_batch, batch))
     {
       last_batch->draw.vbo_count += batch->draw.vbo_count;