added UnmarshalBinary to VertexDecl

Specifically, was added for reading mesh data for bgfx examples. Who
knows if we keep it.. feels like there is a better solution somewhere.
updated lib/bgfx and lib/bx

bkaradzic/bgfx@d1b6cc762c6660240eed8c726d9d545fdfcd78fb
bkaradzic/bx@2ea546dba017e0230fbfda1eb76ac9ef422cd0dc