Fix bug with same gradient on two elements

src/gradient.toit

@@ -62,7 +62,8 @@ class GradientBackground implements Background:
     return hash_
 
   draw canvas/Canvas x/int y/int w/int h/int --autoclipped/bool -> none:
-    if not rendering_: rendering_ = GradientRendering_.get w h this
+    if not rendering_ or rendering_.w_ != w or rendering_.h_ != h:
+      rendering_ = GradientRendering_.get w h this
     rendering_.draw canvas x y --autoclipped=autoclipped
 
   static normalize-angle_ angle/int -> int:

tests/sized-gradients-visualized.toit

@@ -0,0 +1,39 @@
+// Copyright (C) 2024 Toitware ApS.
+// Use of this source code is governed by a Zero-Clause BSD license that can
+// be found in the TESTS_LICENSE file.
+
+// Tests that using the same gradient on two elements with different
+// sizes works as expected.  A new GradientRendering is used for the
+// second element.
+
+import bitmap show *
+import expect show *
+import font show *
+import pixel-display show *
+import pixel-display.gradient show *
+import .png-visualizer
+
+main args:
+  if args.size != 1:
+    print "Usage: script.toit png-basename"
+    exit 1
+  driver := TrueColorPngVisualizer 192 120 args[0] --outline=0x4040ff
+  display := PixelDisplay.true-color driver
+  display.background = 0x808080
+
+  border := SolidBorder --color=0x000000 --width=3
+
+  gradient := GradientBackground --angle=30 --specifiers=[
+      GradientSpecifier --color=0xffff00 10,
+      GradientSpecifier --color=0xffc000 50,
+      GradientSpecifier --color=0x808000 90,
+      ]
+  gradient-element := Div --x=10 --y=10 --w=50 --h=50 --background=gradient --border=border
+  display.add gradient-element
+
+  gradient-2-element := Div --x=70 --y=10 --w=100 --h=100 --background=gradient --border=border
+  display.add gradient-2-element
+
+  display.draw
+
+  driver.write-png