const in ShaderStage that represents vertex and fragment access (gfx-rs#1229)

* const in ShaderStage that represents vertex and fragment access

We can store `ShaderStage`s as const values or members of const structs in all individual cases, but for example `ShaderStage::VERTEX | ShaderStage::FRAGMENT` cannot be stored in a const. This is not very elegant, but would be convenient.

* Correct VERTEX_FRAGMENT definition

VERTEX_FRAGMENT definition now follows directly from the bit representation of `ShaderStage::VERTEX` and `Shaderstage::FRAGMENT` such that it does not need to be independently maintained.

Author: hi-jklmn

wgpu-types/src/lib.rs

@@ -566,6 +566,8 @@ bitflags::bitflags! {
         const VERTEX = 1;
         /// Binding is visible from the fragment shader of a render pipeline.
         const FRAGMENT = 2;
+        /// Binding is visible from the vertex and fragment shaders of a render pipeline.
+        const VERTEX_FRAGMENT = Self::VERTEX.bits | Self::FRAGMENT.bits;
         /// Binding is visible from the compute shader of a compute pipeline.
         const COMPUTE = 4;
     }