plotRGB alpha should be interpolated like r, g, b

[andrewclausen]

I'm not sure if plotRGB is supposed to support alpha (transparency) being a matrix rather than just a scalar.

In any case, whether it actually works is hit and miss. sampleRegular() is used rescale/crop r,g,b but not alpha.

Are pull requests welcome for addressing both issues? That is,

1. fully documenting alpha channel support
2. fixing bugs in this support.