wgpu: Fix gradient shader on vulkan backend

A regression in naga caused a mistranslation of the gradient shader
on the vulkan backend. This caused radial gradients to be rendered
as linear gradients on the vulkan backend. dx12 seems unaffected.

This seems to be cased by a switch statement starting with a default
case Re-order the case statements to avoid this. Looks like it's been
fixed in naga master.

render/wgpu/shaders/gradient.wgsl

@@ -35,12 +35,6 @@ fn main_fragment(in: VertexOutput) -> [[location(0)]] vec4<f32> {
     // Calculate normalized `t` position in gradient, [0.0, 1.0] being the bounds of the ratios.
     var t: f32;
     switch( gradient.gradient_type ){
-        // Linear gradient
-        default: {
-            t = in.uv.x;
-            break;
-        }
-
         // Radial gradient
         case 1: {
             t = length(in.uv * 2.0 - 1.0);
@@ -56,16 +50,16 @@ fn main_fragment(in: VertexOutput) -> [[location(0)]] vec4<f32> {
             t = l / (sqrt(1.0 - gradient.focal_point * gradient.focal_point * d.y * d.y) + gradient.focal_point * d.x);
             break;
         }
-    }
 
-    // Tweak out-of-bounds `t` based on the repeat mode.
-    switch( gradient.repeat_mode ){
-        // Clamp
+        // Linear gradient
         default: {
-            t = clamp(t, 0.0, 1.0);
+            t = in.uv.x;
             break;
         }
+    }
 
+    // Tweak out-of-bounds `t` based on the repeat mode.
+    switch( gradient.repeat_mode ){
         // Repeat
         case 1: {
             t = fract(t);
@@ -85,6 +79,12 @@ fn main_fragment(in: VertexOutput) -> [[location(0)]] vec4<f32> {
             }
             break;
         }
+
+        // Clamp
+        default: {
+            t = clamp(t, 0.0, 1.0);
+            break;
+        }
     }
     t = clamp(t, gradient.ratios[0], gradient.ratios[last]);