Merge sibling <g> nodes with identical attributes

Created by: nthykier

In some cases, gnuplot generates a very suboptimal SVG content of the following pattern:

    <g color="black" fill="none" stroke="currentColor">
    <path d="m82.5 323.3v-4.1" stroke="#000"/>
    </g>
    <g color="black" fill="none" stroke="currentColor">
    <path d="m116.4 323.3v-4.1" stroke="#000"/>
    </g>
    ... repeated 10+ more times here ...
    <g color="black" fill="none" stroke="currentColor">
    <path d="m65.4 72.8v250.5h420v-250.5h-420z" stroke="#000"/>
    </g>

A more optimal pattern would be:

    <g color="black" fill="none" stroke="#000">
    <path d="m82.5 323.3v-4.1"/>
    <path d="m116.4 323.3v-4.1"/>
    ... 10+ more paths here ...
    <path d="m65.4 72.8v250.5h420v-250.5h-420z"/>
    </g>

This patch enables that optimization by handling the merging of two sibling entries that have identical attributes. In the above example that does not solve the rewrite from "currentColor" to "#000" for the stroke attribute. However, the existing code already handles that automatically after the elements have been merged.

This change provides comparable results to --create-groups as shown by the following diagram while being a distinct optimization:

   +----------------------------+-------+--------+
   |           Test             | Size  |  in %  |
   +----------------------------+-------+--------+
   | baseline                   | 17961 |  100%  |
   | baseline + --create-groups | 17418 |  97.0% |
   | patched                    | 16939 |  94.3% |
   | patched + --create-groups  | 16855 |  93.8% |
   +----------------------------+-------+--------+

The image used in the size table above was generated based on the instructions from https://bugs.debian.org/858039#10 with gnuplot 5.2 patchlevel 2. Beyond the test-based "--create-groups", the following scour command-line parameters were used: --enable-id-stripping --enable-comment-stripping
--shorten-ids --indent=none

Note that the baseline was scour'ed repeatedly to stablize the image size.

Signed-off-by: Niels Thykier niels@thykier.net