D3D12 fix for register binding offsets.

When both the vert and frag shaders have a UBO binding, the D3D12
backend was using register offset 0 for both, causing a collision,
and the wrong constant value used in one of the shaders.

The fix is to use the binding offsets computed by the BindGroupLayout,
since they know about all of the bindings, not just the ones computed
for each shader. This made it necessary to defer shader compilation
until pipeline layout creation time (as is done in the Metal backend
for similar reasons).

Finally, those bindings offsets computed by the BGL include an offset
for the CBV, UAV and SRV subgroups, so we must add the same register
offset when assigning the BaseShaderRegister to the descriptor ranges
in the PipelineLayout constructor so that they match.

Bug: dawn:20

Change-Id: I18287bf1c06f06dd61288e12da64752f54634466
Reviewed-on: https://dawn-review.googlesource.com/c/1960
Reviewed-by: Stephen White <senorblanco@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Stephen White <senorblanco@chromium.org>

src/dawn_native/d3d12/ShaderModuleD3D12.h

diff --git a/src/dawn_native/d3d12/ShaderModuleD3D12.h b/src/dawn_native/d3d12/ShaderModuleD3D12.h
index 11065c1..7cafd1c 100644
--- a/src/dawn_native/d3d12/ShaderModuleD3D12.h
+++ b/src/dawn_native/d3d12/ShaderModuleD3D12.h
@@ -20,15 +20,16 @@
 namespace dawn_native { namespace d3d12 {
 
     class Device;
+    class PipelineLayout;
 
     class ShaderModule : public ShaderModuleBase {
       public:
         ShaderModule(Device* device, const ShaderModuleDescriptor* descriptor);
 
-        const std::string& GetHLSLSource() const;
+        const std::string GetHLSLSource(PipelineLayout* layout) const;
 
       private:
-        std::string mHlslSource;
+        std::vector<uint32_t> mSpirv;
     };
 
 }}  // namespace dawn_native::d3d12