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/d3dcompile/compile_windows.go

@@ -0,0 +1,107 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+package d3dcompile
+
+import (
+	"fmt"
+	"unsafe"
+
+	"syscall"
+
+	gunsafe "gioui.org/internal/unsafe"
+
+	"golang.org/x/sys/windows"
+)
+
+var (
+	d3dcompiler_47 = windows.NewLazySystemDLL("d3dcompiler_47.dll")
+
+	__D3DCompile = d3dcompiler_47.NewProc("D3DCompile")
+)
+
+type _IUnknownVTbl struct {
+	QueryInterface uintptr
+	AddRef         uintptr
+	Release        uintptr
+}
+
+type _ID3DBlob struct {
+	vtbl *struct {
+		_IUnknownVTbl
+		GetBufferPointer uintptr
+		GetBufferSize    uintptr
+	}
+}
+
+func D3DCompile(src []byte, entryPoint, target string) ([]byte, error) {
+	var (
+		code   *_ID3DBlob
+		errors *_ID3DBlob
+	)
+	entryPoint0 := []byte(entryPoint + "\x00")
+	target0 := []byte(target + "\x00")
+	r, _, _ := __D3DCompile.Call(
+		uintptr(unsafe.Pointer(&src[0])),
+		uintptr(len(src)),
+		0, // pSourceName
+		0, // pDefines
+		0, // pInclude
+		uintptr(unsafe.Pointer(&entryPoint0[0])),
+		uintptr(unsafe.Pointer(&target0[0])),
+		0, // Flags1
+		0, // Flags2
+		uintptr(unsafe.Pointer(&code)),
+		uintptr(unsafe.Pointer(&errors)),
+	)
+	var compileErr string
+	if errors != nil {
+		compileErr = string(errors.data())
+		_IUnknownRelease(unsafe.Pointer(errors), errors.vtbl.Release)
+	}
+	if r != 0 {
+		return nil, fmt.Errorf("D3D11Compile: %#x: %s", r, compileErr)
+	}
+	bytecode := code.data()
+	cp := make([]byte, len(bytecode))
+	copy(cp, bytecode)
+	_IUnknownRelease(unsafe.Pointer(code), code.vtbl.Release)
+	return cp, nil
+}
+
+func (b *_ID3DBlob) GetBufferPointer() uintptr {
+	ptr, _, _ := syscall.Syscall(
+		b.vtbl.GetBufferPointer,
+		1,
+		uintptr(unsafe.Pointer(b)),
+		0,
+		0,
+	)
+	return ptr
+}
+
+func (b *_ID3DBlob) GetBufferSize() uintptr {
+	sz, _, _ := syscall.Syscall(
+		b.vtbl.GetBufferSize,
+		1,
+		uintptr(unsafe.Pointer(b)),
+		0,
+		0,
+	)
+	return sz
+}
+
+func (b *_ID3DBlob) data() []byte {
+	data := gunsafe.SliceOf(b.GetBufferPointer())
+	n := int(b.GetBufferSize())
+	return data[:n:n]
+}
+
+func _IUnknownRelease(obj unsafe.Pointer, releaseMethod uintptr) {
+	syscall.Syscall(
+		releaseMethod,
+		1,
+		uintptr(obj),
+		0,
+		0,
+	)
+}