gsk: Respect max element vertices limitation

We are pretty good at batching commands now, and we can easily
produce batches that exceed the maximum number of elements per
draw call that the hw can handle. Query that number, and respect
it when merging batches.

This fixes the rendering of the overview map in GtkSourceView.

gsk/ngl/gsknglcommandqueue.c

@@ -444,9 +444,12 @@ gsk_ngl_command_queue_new (GdkGLContext       *context,
   else
     self->uniforms = gsk_ngl_uniform_state_new ();
 
-  /* Determine max texture size immediately and restore context */
+  /* Determine max texture size and other limitations immediately
+   * and restore context
+   */
   gdk_gl_context_make_current (context);
   glGetIntegerv (GL_MAX_TEXTURE_SIZE, &self->max_texture_size);
+  glGetIntegerv (GL_MAX_ELEMENTS_VERTICES, &self->max_elements_vertices);
 
   return g_steal_pointer (&self);
 }
@@ -617,6 +620,7 @@ 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 <= self->max_elements_vertices &&
       snapshots_equal (self, last_batch, batch))
     {
       last_batch->draw.vbo_count += batch->draw.vbo_count;

gsk/ngl/gsknglcommandqueueprivate.h

@@ -231,6 +231,12 @@ struct _GskNglCommandQueue
    */
   int max_texture_size;
 
+  /* Discovered max element count. We must not create batches that contain
+   * more vertices than this number.
+   */
+
+  int max_elements_vertices;
+
   /* The index of the last batch in @batches, which may not be the element
    * at the end of the array, as batches can be reordered. This is used to
    * update the "next" index when adding a new batch.