Mesh pipeline key requires strip index format bits

BaseMeshPipelineKey and Mesh2dPipelineKey now have STRIP_INDEX_FORMAT_* bits because strip_index_format will be required by wgpu and primitive restart is always enabled.

The strip index format bits in the mesh pipeline key must match the mesh index format for indexed strip topologies (For non-indexed strip topologies, the bits don't matter), and must be STRIP_INDEX_FORMAT_NONE for non-strip topologies. The from_primitive_topology method of mesh pipeline key has been changed to from_primitive_topology_and_strip_index to handle it and RenderMesh now has an index_format method.

In 0.18:

let key = MeshPipelineKey::from_primitive_topology(render_mesh.primitive_topology());

In 0.19:

let key = MeshPipelineKey::from_primitive_topology_and_strip_index(
    render_mesh.primitive_topology(),
    render_mesh.index_format(),
);