internal/cmd/convertshaders: replace fxc.exe with D3DCompile

D3DCompile successfully compiles shaders fxc.exe doesn't. As a bonus
the DirectX SDK is no longer required (it includes fxc.exe).

Signed-off-by: Elias Naur <mail@eliasnaur.com>

internal/cmd/convertshaders/hlsl_noop.go

@@ -0,0 +1,9 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+// +build !windows
+
+package main
+
+func compileHLSL(src, entry, profile string) ([]byte, error) {
+	return nil, nil
+}