rsvg-convert: fix dimensions of SVG output

The RsvgDimensionData structure uses pixels as unit for drawing
dimensions (see librsvg/rsvg.h), but the default unit for SVG documents
in cairo is pt; thus, when exporting to SVG with rsvg-convert, the
output drawing results scaled up compared to the original one.

Since rsvg already converted input units to pixels, it's safe to assume
the SVG output is always in pixels, so set the SVG surface units to
pixels to keep the original dimensions.

Bump cairo dependency to 1.5.12 as cairo_svg_surface_set_document_unit()
was not available before that version.

Fixes GNOME/librsvg#365
3 jobs for <span class="ref-name">rsvg-convert-fix-svg-output-units</span> in 8 minutes and 13 seconds (queued for 18 minutes and 39 seconds)
Status Job ID Name Coverage
  Test
passed #130676
fedora test

00:07:51

 
  Lint
manual #130678
allowed to fail manual
clippy
failed #130677
rustfmt

00:00:22

 
Name Stage Failure
failed
rustfmt Lint
+        let offset = baseline
+ + values
+ .baseline_shift
+ .0
+ .normalize(values, &draw_ctx.get_view_params());

if values.text_gravity_is_vertical() {
draw_ctx.draw_pango_layout(&layout, values, *x + offset, *y, clipping)?;
ERROR: Job failed: exit code 1