Replace vec4 single channel reads with block read
Replace vec4 single channel reads with block read

When converting a quaternion to angles we're doing separate channel
reads on two different vectors. These are more expensive than a single
block read, so let's replace them with calls to graphene_vec4_to_float()
instead. The readability suffers slightly, but it's still a win.

ebassi/graphene,ebassi/graphene