Fix setting mesh blend shape properties in dummy mesh storage

[nikitalita]

When using the dummy rendering server, ImporterMesh::add_surface() fails if the mesh has blend shapes:

ERROR: Condition "blend_shape_count != (uint32_t)mesh_get_blend_shape_count(p_mesh)" is true. Returning: Array()
	at: mesh_surface_get_blend_shape_arrays (servers/rendering_server.cpp:1774)
ERROR: Condition "p_blend_shapes.size() != blend_shapes.size()" is true.
	at: add_surface (scene/resources/3d/importer_mesh.cpp:68)

This causes various things, like the scene importers, to produce inaccurate scenes when running headless.

This is caused by the dummy mesh storage not actually setting and getting the blend_shape_count/blend_shape_mode. Implementing those fixes the issue.