fuzzel: refactor configure options

The current configure options exposed emulates the `choices` type of the
SVG and PNG backends from upstream meson_options with a freeform string,
however this is error prone and require accompanying validation which
unnecessarily complicates the derivation. Two alternatives were considered:

1. Splits the possible choices into separate options nad perform assertions to
   ensure only one is selected.
2. Expose a set of sane default configure options, and structure the derivation
   around this subset. Users who wish to customize the choice used can do so
   using `override` or `overrideAttrs`.

Option 1 is not sustainable for large amounts of options which require large
amount of assertions just for ensuring uniqueness of selection, and adds
complexity to the configure phase. Option 2 minimally impairs the customizablity
of the derivation and was chosen in part to address option checking[1].

[1] - https://github.com/NixOS/nixpkgs/pull/141836#discussion_r739910095

pkgs/applications/misc/fuzzel/default.nix

@@ -13,14 +13,16 @@
 , tllist
 , fcft
 , enableCairo ? true
-, withPNGBackend ? "libpng"
-, withSVGBackend ? "librsvg"
-  # Optional dependencies
+, svgSupport ? true
+, pngSupport ? true
+# Optional dependencies
 , cairo
 , librsvg
 , libpng
 }:
 
+assert svgSupport -> enableCairo;
+
 stdenv.mkDerivation rec {
   pname = "fuzzel";
   version = "1.7.0";
@@ -49,15 +51,15 @@ stdenv.mkDerivation rec {
     tllist
     fcft
   ] ++ lib.optional enableCairo cairo
-  ++ lib.optional (withPNGBackend == "libpng") libpng
-  ++ lib.optional (withSVGBackend == "librsvg") librsvg;
+    ++ lib.optional pngSupport libpng
+    ++ lib.optional svgSupport librsvg;
 
   mesonBuildType = "release";
 
   mesonFlags = [
     "-Denable-cairo=${if enableCairo then "enabled" else "disabled"}"
-    "-Dpng-backend=${withPNGBackend}"
-    "-Dsvg-backend=${withSVGBackend}"
+    "-Dpng-backend=${if pngSupport then "libpng" else "none"}"
+    "-Dsvg-backend=${if svgSupport then "librsvg" else "none"}"
   ];
 
   CFLAGS = "-Wno-error=comment"; # https://gitlab.gnome.org/GNOME/librsvg/-/issues/856