io/semantic: add package for adding semantic descriptions to UI components

Software such as screen readers require semantic descriptions of user
interfaces to effectively present and interact with them. Package
semantic, combined with the existing package clip provide the operations
for Gio programs to describe themselves.

This change implements the semantic package and the routing changes for
accessing semantic trees; follow-ups add semantic information to widgets
and implement mapping semantic tree to platform representations.

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

internal/ops/ops.go

@@ -24,7 +24,7 @@ type Ops struct {
 	nextStateID int
 
 	macroStack stack
-	stacks     [4]stack
+	stacks     [5]stack
 }
 
 type OpType byte
@@ -63,6 +63,11 @@ const (
 	TypeCursor
 	TypePath
 	TypeStroke
+	TypeSemanticLabel
+	TypeSemanticDesc
+	TypeSemanticClass
+	TypeSemanticSelected
+	TypeSemanticDisabled
 )
 
 type StackID struct {
@@ -98,6 +103,7 @@ const (
 	ClipStack StackKind = iota
 	TransStack
 	PassStack
+	MetaStack
 )
 
 const (
@@ -107,34 +113,39 @@ const (
 )
 
 const (
-	TypeMacroLen           = 1 + 4 + 4
-	TypeCallLen            = 1 + 4 + 4
-	TypeDeferLen           = 1
-	TypePushTransformLen   = 1 + 4*6
-	TypeTransformLen       = 1 + 1 + 4*6
-	TypePopTransformLen    = 1
-	TypeRedrawLen          = 1 + 8
-	TypeImageLen           = 1
-	TypePaintLen           = 1
-	TypeColorLen           = 1 + 4
-	TypeLinearGradientLen  = 1 + 8*2 + 4*2
-	TypePassLen            = 1
-	TypePopPassLen         = 1
-	TypePointerInputLen    = 1 + 1 + 1*2 + 2*4 + 2*4
-	TypeClipboardReadLen   = 1
-	TypeClipboardWriteLen  = 1
-	TypeKeyInputLen        = 1 + 1
-	TypeKeyFocusLen        = 1 + 1
-	TypeKeySoftKeyboardLen = 1 + 1
-	TypeSaveLen            = 1 + 4
-	TypeLoadLen            = 1 + 4
-	TypeAuxLen             = 1
-	TypeClipLen            = 1 + 4*4 + 1 + 1
-	TypePopClipLen         = 1
-	TypeProfileLen         = 1
-	TypeCursorLen          = 1 + 1
-	TypePathLen            = 8 + 1
-	TypeStrokeLen          = 1 + 4
+	TypeMacroLen            = 1 + 4 + 4
+	TypeCallLen             = 1 + 4 + 4
+	TypeDeferLen            = 1
+	TypePushTransformLen    = 1 + 4*6
+	TypeTransformLen        = 1 + 1 + 4*6
+	TypePopTransformLen     = 1
+	TypeRedrawLen           = 1 + 8
+	TypeImageLen            = 1
+	TypePaintLen            = 1
+	TypeColorLen            = 1 + 4
+	TypeLinearGradientLen   = 1 + 8*2 + 4*2
+	TypePassLen             = 1
+	TypePopPassLen          = 1
+	TypePointerInputLen     = 1 + 1 + 1*2 + 2*4 + 2*4
+	TypeClipboardReadLen    = 1
+	TypeClipboardWriteLen   = 1
+	TypeKeyInputLen         = 1 + 1
+	TypeKeyFocusLen         = 1 + 1
+	TypeKeySoftKeyboardLen  = 1 + 1
+	TypeSaveLen             = 1 + 4
+	TypeLoadLen             = 1 + 4
+	TypeAuxLen              = 1
+	TypeClipLen             = 1 + 4*4 + 1 + 1
+	TypePopClipLen          = 1
+	TypeProfileLen          = 1
+	TypeCursorLen           = 1 + 1
+	TypePathLen             = 8 + 1
+	TypeStrokeLen           = 1 + 4
+	TypeSemanticLabelLen    = 1
+	TypeSemanticDescLen     = 1
+	TypeSemanticClassLen    = 2
+	TypeSemanticSelectedLen = 2
+	TypeSemanticDisabledLen = 2
 )
 
 func (op *ClipOp) Decode(data []byte) {
@@ -354,12 +365,17 @@ func (t OpType) Size() int {
 		TypeCursorLen,
 		TypePathLen,
 		TypeStrokeLen,
+		TypeSemanticLabelLen,
+		TypeSemanticDescLen,
+		TypeSemanticClassLen,
+		TypeSemanticSelectedLen,
+		TypeSemanticDisabledLen,
 	}[t-firstOpIndex]
 }
 
 func (t OpType) NumRefs() int {
 	switch t {
-	case TypeKeyInput, TypeKeyFocus, TypePointerInput, TypeProfile, TypeCall, TypeClipboardRead, TypeClipboardWrite, TypeCursor:
+	case TypeKeyInput, TypeKeyFocus, TypePointerInput, TypeProfile, TypeCall, TypeClipboardRead, TypeClipboardWrite, TypeCursor, TypeSemanticLabel, TypeSemanticDesc:
 		return 1
 	case TypeImage:
 		return 2
@@ -426,6 +442,8 @@ func (t OpType) String() string {
 		return "Path"
 	case TypeStroke:
 		return "Stroke"
+	case TypeSemanticLabel:
+		return "SemanticDescription"
 	default:
 		panic("unknown OpType")
 	}