Enable support for high-bitdepth gradients to make better use of 16-bit png export

Steps to reproduce:

  • open Inkscape
  • Create a gradient
  • Export as 16-bit png

What happened?

  • Banding is visible, and the smallest normailzed color-step is 1/256.

What should have happened?

  • Smallest normalized color-step should be 1/65536 (higher if the gradient is not 65536 pixels long)gray_gradient.svggray_gradient.

Inkscape Version and Operating System:

  • Inkscape Version: 1.0beta2 (2b71d25d45, 2019-12-03) compiled from source
  • Operating System: Xubuntu
  • Operating System version: 18.04.4 LTS