logo: Make source SVGs self-contained

This makes these SVGs a bit easier to reason about.  More
concretely, this will allow us to switch from Inkscape to librsvg
for the conversion: the latter is faster and has some other useful
properties, but doesn't support using xlink to point to a separate
file as these were doing.

This change is nearly NFC: it'd be NFC if the image processing were
mathematically exact.  In reality the old and new output images look
identical to my eye, but examining with `sng` (like in the previous
commit upgrading Inkscape) confirms some pixels have slightly
different values.  Like with that commit, we leave those changes out
for now, to bundle them with substantive changes coming later in
this commit series.

Author: gnprice

assets/app-icons/zulip-icon-circle-beta.svg

@@ -48,28 +48,24 @@ strip-nondeterminism --version >/dev/null 2>&1 \
     || die "Need strip-nondeterminism -- try 'apt install strip-nondeterminism'."
 
 
-# This should point to a zulip.git worktree.
-zulip_root="${root_dir%/*}"/zulip
+# Suppose (for comments below) this points to a zulip.git worktree.
+# zulip_root="${root_dir%/*}"/zulip
 
 # White Z in gradient-colored circle.
-src_icon_circle="${zulip_root}"/static/images/logo/zulip-icon-circle.svg
+# src_icon_circle="${zulip_root}"/static/images/logo/zulip-icon-circle.svg
 
 # White Z in gradient-colored circle with BETA banner.
-# Contains a link to the equivalent of ${src_icon_circle}.
+# Contains a near-verbatim copy of ${src_icon_circle}.
 src_icon_circle_beta="${root_dir}"/assets/app-icons/zulip-icon-circle-beta.svg
 
 # White Z in gradient-colored square, full-bleed.
 # src_icon_square="${zulip_root}"/static/images/logo/zulip-icon-square.svg
 
 # White Z in gradient-colored square, full-bleed, with BETA banner.
-# Contains a link to the equivalent of ${src_icon_square}.
+# Contains a near-verbatim copy of ${src_icon_square}.
 src_icon_square_beta="${root_dir}"/assets/app-icons/zulip-icon-square-beta.svg
 
 
-[ -r "${src_icon_circle}" ] \
-    || die "Expected Zulip worktree at: ${zulip_root}"
-
-
 make_one_ios_app_icon() {
     # SET BY CALLER: contents iconset
     local size_pt="$1" scale="$2" idiom="$3"