scales::alpha() performance and color format ("character" or "native")

[zeehio]

This pull request is on top of:

• https://github.com/thomasp85/farver/pull/40

It uses the improvements on farver to make scales::alpha() much faster, avoiding the intermediate step of the numeric matrix.

Besides, it adds an additional parameter color_fmt to choose between a "character" output format or the "native" output format, used in nativeRaster objects.

Proposed NEWS entry (not added to avoid trivial conflicts):

  * `alpha()` performance improvements. Added support for `native` color format (#, @zeehio)

Related to:

• https://github.com/tidyverse/ggplot2/issues/4989