LateNight: fix xfader icons in samplers and aux units

[ronso0]

The samplers xfader icns were only half the size, so doubling the size and setting the 1:1 viewbox attribute does not work for icons set in xml templates via

<State>
  <Number>2</Number>
  <Pressed scalemode="STRETCH_ASPECT">skins:LateNight/<Variable name="BtnScheme"/>/buttons/btn__xfader_sampler_right.svg</Pressed>
  <Unpressed scalemode="STRETCH_ASPECT">skins:LateNight/<Variable name="BtnScheme"/>/buttons/btn__xfader_sampler_right.svg</Unpressed>
</State>

it has the contrary effect, exactly what daschuer noticed here #12364 (comment)

@m0dB please check if that looks okay on your machine.

Addionally, I fixed blurred edges caused by a pixel offset in double-size icons (Aux xfader icons).

[m0dB]

At a first glance it looks good, but some icons are now huge: 253px x 300px . Is this intentional?

[ronso0]

Oh, I didn't look at the diff. The width and height attributes were removed (now both "0") as well as the closing </svg>.
I'll fix it.

[ronso0]

Fixed. @m0dB please take anoter look.

[m0dB]

LGTM, merged!