Canvas color should ignore background color transparency

[William Swanson]

Requested functionality:
When the user selects a background color in the
Document Preferences dialog, the drawing canvas' color
should match the RGB component and ignore the alpha
component.

Current functionality:
In the current implementation, the drawing canvas'
color is the page's background color blended with pure
white according to the background's alpha component.

Motivation:
When a user creates a transparent image, they typically
intend to use it over a colored background of some
sort. For example, a web designer may wish to place a
transparent .png over a mostly-blue page background. As
they design the image in Inkscape, their canvas color
should approximate the the target's background color,
allowing the designer to make appropriate color
decisions. Then, when they export the image, the
background should be fully transparent according to the
original goal of making a transparent image.

The most reasonable way for the user to express this
intent is to set up a fully-transparent page background
with a color-value that matches the target medium's
background color. This way, the image saves with full
transparency, yet the user sees an appropriate
background color as they design. Unfortunately, the
current blend-to-white functionality prevents this
behavior. If the user wishes to create a transparent
image, they are forced to draw against a white
background. Causing the canvas color to ignore the
background color's transparency should make Inkscape
much more usable in these practical situations.

[Rwst (rwst)]

implemented in cvs

[qubodup (qubodup)]

Rwst: is it?

[qubodup (qubodup)]

Short summary: The background's (color) setting's ALPHA value should not affect what you see in Inkscape as the background, it should ONLY affect the background of the exported image!
Some of use prefer dark backgrounds but still want transparent exports :) currently one has to change the alpha value of the background color to change the GUI's background color to anything but white.

[su_v (suv-lp)]

Implemented as fix for Bug #169623 “Canvas color option”
<https://bugs.launchpad.net/inkscape/+bug/169623>