[librsvg/librsvg-2.50] Really make the initial viewport have its upper-left corner at (0, 0)

[Federico Mena Quintero <federico src gnome org>]

commit 81ec44d17c687ac32c1b790c276898a31c13a82f
Author: Federico Mena Quintero <federico gnome org>
Date:   Mon Sep 28 19:29:23 2020 -0500

    Really make the initial viewport have its upper-left corner at (0, 0)

 rsvg_internals/src/drawing_ctx.rs | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)
---
diff --git a/rsvg_internals/src/drawing_ctx.rs b/rsvg_internals/src/drawing_ctx.rs
index b7ded244..ff0aa5a8 100644
--- a/rsvg_internals/src/drawing_ctx.rs
+++ b/rsvg_internals/src/drawing_ctx.rs
@@ -205,10 +205,17 @@ impl DrawingCtx {
         // "... the initial viewport coordinate system (and therefore
         // the initial user coordinate system) must have its origin at
         // the top/left of the viewport"
+
+        // Translate so (0, 0) is at the viewport's upper-left corner.
+        cr.translate(viewport.x0, viewport.y0);
+        let transform = Transform::from(cr.get_matrix());
+
+        // Per the spec, so the viewport has (0, 0) as upper-left.
+        let viewport = viewport.translate((-viewport.x0, -viewport.y0));
         let vbox = ViewBox::from(viewport);
 
         let initial_viewport = Viewport {
-            transform: Transform::from(cr.get_matrix()),
+            transform,
             vbox,
         };