Fix body rendering when border is applied

The block reflower resolves width, height, margins, and insets (`top`,
`right`, `bottom`, `left`). Any case of unresolved margin values should
be fixed in the reflower, so `length_in_pt` and `float` casts are
unnecessary here.

Fixes #2939

src/Renderer/Block.php

@@ -32,12 +32,10 @@ function render(Frame $frame)
         [$x, $y, $w, $h] = $frame->get_border_box();
 
         if ($node->nodeName === "body") {
-            $h = $frame->get_containing_block("h") - (float)$style->length_in_pt([
-                        $style->margin_top,
-                        $style->border_top_width,
-                        $style->border_bottom_width,
-                        $style->margin_bottom],
-                    (float)$style->length_in_pt($style->width));
+            // Margins should be fully resolved at this point
+            $mt = $style->margin_top;
+            $mb = $style->margin_bottom;
+            $h = $frame->get_containing_block("h") - $mt - $mb;
         }
 
         $border_box = [$x, $y, $w, $h];