Check-in [91d1a66aae]

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Remove the option "-unit".
The given unit is used to set the unit of the intermediate set of polygons.

The final unit will be pixels anyway, undependent on any intermediate unit.
Image width and height is in pixel.
There is no other unit available for image width or height.

The current implementation does the following:

   *   the intermediate representation is scaled in the unit given by "-unit" using the current dpi value. The result depends on the unit used in the svg, to specified unit and the dpi value (96 or specified by -dpi).
   *   the size of the image is evaluated by transforming the given unit into pixel
   *   the image data is rendered by assuming (not transforming: 1 inch is set to 1 pixel) that unit is pixel. If it was not pixel, a scale effect is shown by accident.

Here is an example from the core list:

    set data {<svg xmlns="http://www.w3.org/2000/svg" width="100" height="100"><circle fill="yellow" stroke="red" cx="50" cy="50" r="30" /></svg>}
    set im [image create photo -data $data]
    pack [button .b -image $im]
    $im configure -format {svg -unit px}  ; # no change (this is the default)
    $im configure -format {svg -unit mm}  ; # image gets scaled to a smaller image, but image size stays the same

    # Set inner scale to one:
    $im configure -format {svg -unit in -dpi 1}
    image width $im
    -> 100
    # with the same result as no format option at all (e.g. "svg -unit px -dpi 96")
    
    # Set inner scale to 0.1, but keep same size
    $im configure -format {svg -unit in -dpi 10}
    image width $im
    -> 100
    # but image data is scale by factor 0.1
    
    # The standard way to get an image scaled by 0.1 but with size set to 100x100 is as follows:
    set im2 [image create photo -data $data -format {svg -scale 0.1} -width 100 -height 100]
    pack [button .b2 -image $im2]
</verbatim>

## Add a possibility to scale to a given width or height in pixels

A common usage of svg is to render button images for a given button height or width.
Buttons of applications may have all the same height or width when aranged in button lines aranged horizontally or vertically.

The current solution to render a svg to a given height "$height" is: