Relax the buffer role change restrictions in GL

[kvark]

I think the index_buffer_role_change restriction should still allow transfers (i.e. TRANSFER_XXX). Here are the steps to get there:

First, in add_memory_type function we'd need to have something like:

let restricted_usage = buffer::Usage::INDEX | buffer::Usage::TRANSFER_SRC | buffer::Usage::TRANSFER_DST;
memory_types.push((memory_type, MemoryUsage::Buffer(restricted_usage)));
memory_types.push((
    memory_type,
    MemoryUsage::Buffer(buffer::Usage::all() - restricted_usage),
));

Secondly, in queue.rs when CopyBufferToBuffer and friends are implemented, they need to respect the bind points of the buffers. Currently, they use COPY_READ_BUFFER and COPY_WRITE_BUFFER unconditionally. Instead, these would need to be passed in.