Colormap alpha support

Looks like `transfer_functions.shade()` only allows dynamic alpha when the `cmap` is a single color. That is, if an arbitrary RGBA colormap is passed as `cmap`, the alpha value of that colormap is ignored and a global alpha value is applied to the RGB values pulled from the colormap. Is this the intended behavior?

Start with transfer_functions.shade() and trace how an arbitrary RGBA cmap is handled alongside the global alpha value. Confirm the intended behavior for colormap alpha, then add focused regression coverage; the work is done when the selected alpha behavior is implemented and verified.

Written by the indexing model from the issue text.