decoder: Use image-rs, libheif-rs, librsvg

This adds proper support for a lot of image formats including SVGs.
It will be the basis for many features to come. The technical core
is an asynchronous renderer that can arrange textures in smaller tiles
which might be generated on demand.

Closes #34, #40, #41, #49

Basis for fixing the following issues #7, #27, #36, #43, #68, #71

src/meson.build

@@ -67,4 +67,17 @@ cargo_doc = custom_target(
     cargo, 'doc',
     cargo_options + ['--no-deps', '--document-private-items'],
   ],
-)
+)
+
+test(
+  'cargo-test',
+  cargo,
+  args: [
+    'test',
+    cargo_options,
+    '--',
+    '--test-threads=1',
+  ],
+  is_parallel: false,
+  timeout: 1800,
+)