This fixes an invalid length being reported from
`float32_array_to_slice` (which used the byte length), and also to
generalize getting data from a JS array buffer view, to reduce code
duplication.
The pending type safety issues, like where we could send a `UInt16Array`
where we expect a `Float32` one, should be solved by IDL bindings in
some cases, like `uniform[n]fv` or `vertexAttrib[n]fv`, and with extra
checks in others, like in the pending `texImage2D(..., ArrayBufferView)`.
r? @jdm
Source-Repo: https://github.com/servo/servo
Source-Revision: 4ab1cdece6f594512a0ae9605ff5ebee630a70ca
56fec2ca63