Fix inefficient upload in Mobile Shadows

Clustered performs the following shadow rendering steps

1. Process objects [0; 10) for cascade 0.
2. Process objects [10; 30) for cascade 1.
3. Process objects [30; 100) for cascade 2.
4. Upload objects [0; 100) to GPU.
5. Draw all cascades.

Mobile was supposed to be doing the same, but instead was doing:

1. Process objects [0; 10) for cascade 0.
2. Upload objects [0; 10) to GPU.
3. Process objects [10; 30) for cascade 1.
4. Upload objects [0; 30) to GPU.
5. Process objects [30; 100) for cascade 2.
6. Upload objects [0; 100) to GPU.
7. Draw all cascades.

That is, always reuploaded everything from scratch.
Therefore it pointlessly (and with geometric growth) wasted BW.

servers/rendering/renderer_rd/forward_mobile/render_forward_mobile.cpp

@@ -1426,6 +1426,7 @@ void RenderForwardMobile::_render_shadow_begin() {
 	_update_render_base_uniform_set();
 
 	render_list[RENDER_LIST_SECONDARY].clear();
+	scene_state.instance_data[RENDER_LIST_SECONDARY].clear();
 }
 
 void RenderForwardMobile::_render_shadow_append(RID p_framebuffer, const PagedArray<RenderGeometryInstance *> &p_instances, const Projection &p_projection, const Transform3D &p_transform, float p_zfar, float p_bias, float p_normal_bias, bool p_use_dp, bool p_use_dp_flip, bool p_use_pancake, float p_lod_distance_multiplier, float p_screen_mesh_lod_threshold, const Rect2i &p_rect, bool p_flip_y, bool p_clear_region, bool p_begin, bool p_end, RenderingMethod::RenderInfo *p_render_info, const Transform3D &p_main_cam_transform) {