wgpu: Fix some invalid WGSL

naga is currently lenient about some things which are invalid by
the WGSL spec. The Tint WGSL compiler disagreed, so these shaders
failed to compile in Chrome.

render/wgpu/shaders/bitmap.wgsl

@@ -10,10 +10,10 @@ var<uniform> textureTransforms: TextureTransforms;
 [[group(2), binding(1)]]
 var texture: texture_2d<f32>;
 [[group(3), binding(0)]]
-var sampler: sampler;
+var texture_sampler: sampler;
 
 [[stage(vertex)]]
-fn main(in: VertexInput) -> VertexOutput {
+fn main_vertex(in: VertexInput) -> VertexOutput {
     let matrix = textureTransforms.matrix;
     let uv = (mat3x3<f32>(matrix[0].xyz, matrix[1].xyz, matrix[2].xyz) * vec3<f32>(in.position, 1.0)).xy;
     let pos = globals.view_matrix * transforms.world_matrix * vec4<f32>(in.position.x, in.position.y, 0.0, 1.0);
@@ -21,8 +21,8 @@ fn main(in: VertexInput) -> VertexOutput {
 }
 
 [[stage(fragment)]]
-fn main(in: VertexOutput) -> [[location(0)]] vec4<f32> {
-    var color: vec4<f32> = textureSample(texture, sampler, in.uv);
+fn main_fragment(in: VertexOutput) -> [[location(0)]] vec4<f32> {
+    var color: vec4<f32> = textureSample(texture, texture_sampler, in.uv);
     // Texture is premultiplied by alpha.
     // Unmultiply alpha, apply color transform, remultiply alpha.
     if( color.a > 0.0 ) {

render/wgpu/shaders/color.wgsl

@@ -6,13 +6,13 @@ struct VertexOutput {
 };
 
 [[stage(vertex)]]
-fn main(in: VertexInput) -> VertexOutput {
+fn main_vertex(in: VertexInput) -> VertexOutput {
     let pos: vec4<f32> = globals.view_matrix * transforms.world_matrix * vec4<f32>(in.position.x, in.position.y, 0.0, 1.0);
     return VertexOutput(pos, in.color);
 }
 
 [[stage(fragment)]]
-fn main(in: VertexOutput) -> [[location(0)]] vec4<f32> {
+fn main_fragment(in: VertexOutput) -> [[location(0)]] vec4<f32> {
     let out = in.color * transforms.mult_color + transforms.add_color;
     return output(out);
 }

render/wgpu/shaders/gradient.wgsl

@@ -22,7 +22,7 @@ var<uniform> textureTransforms: TextureTransforms;
 var<storage> gradient: Gradient;
 
 [[stage(vertex)]]
-fn main(in: VertexInput) -> VertexOutput {
+fn main_vertex(in: VertexInput) -> VertexOutput {
     let matrix = textureTransforms.matrix;
     let uv = (mat3x3<f32>(matrix[0].xyz, matrix[1].xyz, matrix[2].xyz) * vec3<f32>(in.position, 1.0)).xy;
     let pos = globals.view_matrix * transforms.world_matrix * vec4<f32>(in.position.x, in.position.y, 0.0, 1.0);
@@ -30,7 +30,7 @@ fn main(in: VertexInput) -> VertexOutput {
 }
 
 [[stage(fragment)]]
-fn main(in: VertexOutput) -> [[location(0)]] vec4<f32> {
+fn main_fragment(in: VertexOutput) -> [[location(0)]] vec4<f32> {
     let last = gradient.num_colors - 1u;
 
     // Calculate normalized `t` position in gradient, [0.0, 1.0] being the bounds of the ratios.
@@ -90,11 +90,11 @@ fn main(in: VertexOutput) -> [[location(0)]] vec4<f32> {
     t = clamp(t, gradient.ratios[0], gradient.ratios[last]);
 
     // Find the two gradient colors bordering our position.
-    var j: u32 = 1u;
-    for( var j: u32 = 1u; t > gradient.ratios[j]; j = j + 1u) {
+    var j: u32;
+    for( j = 1u; t > gradient.ratios[j]; j = j + 1u) {
         // Noop
     }
-    var i: u32 = j - 1u;
+    let i = j - 1u;
 
     // Lerp between the two colors.
     let a = (t - gradient.ratios[i]) / (gradient.ratios[j] - gradient.ratios[i]);

render/wgpu/src/pipelines.rs

@@ -212,12 +212,12 @@ fn create_pipeline_descriptor<'a>(
         layout: Some(pipeline_layout),
         vertex: wgpu::VertexState {
             module: vertex_shader,
-            entry_point: "main",
+            entry_point: "main_vertex",
             buffers: vertex_buffer_layout,
         },
         fragment: Some(wgpu::FragmentState {
             module: fragment_shader,
-            entry_point: "main",
+            entry_point: "main_fragment",
             targets: color_target_state,
         }),
         primitive: wgpu::PrimitiveState {