Added the BackgroundParent interface

Parents are now able to draw backgrounds for their children. This
means we can now have elements inside other elements that aren't
restricted to one background color.

elements/containers/container.go

@@ -216,6 +216,14 @@ func (element *Container) NotifyMinimumSizeChange (child tomo.Element) {
 	element.core.DamageAll()
 }
 
+// DrawBackground draws a portion of the container's background pattern within
+// the specified bounds. The container will not push these changes.
+func (element *Container) DrawBackground (bounds image.Rectangle) {
+	element.core.DrawBackgroundBounds (
+		element.theme.Pattern(tomo.PatternBackground, tomo.State { }),
+		bounds)
+}
+
 // SetTheme sets the element's theme.
 func (element *Container) SetTheme (new tomo.Theme) {
 	if new == element.theme.Theme { return }

elements/containers/document.go

@@ -213,6 +213,14 @@ func (element *DocumentContainer) NotifyMinimumSizeChange (child tomo.Element) {
 	element.core.DamageAll()
 }
 
+// DrawBackground draws a portion of the container's background pattern within
+// the specified bounds. The container will not push these changes.
+func (element *DocumentContainer) DrawBackground (bounds image.Rectangle) {
+	element.core.DrawBackgroundBounds (
+		element.theme.Pattern(tomo.PatternBackground, tomo.State { }),
+		bounds)
+}
+
 // NotifyFlexibleHeightChange notifies the parent that the parameters
 // affecting a child's flexible height have changed. This method is
 // expected to be called by flexible child element when their content

elements/containers/scroll.go

@@ -135,6 +135,14 @@ func (element *ScrollContainer) NotifyScrollBoundsChange (child tomo.Scrollable)
 	}
 }
 
+// DrawBackground draws a portion of the container's background pattern within
+// the specified bounds. The container will not push these changes.
+func (element *ScrollContainer) DrawBackground (bounds image.Rectangle) {
+	element.core.DrawBackgroundBounds (
+		element.theme.Pattern(tomo.PatternBackground, tomo.State { }),
+		bounds)
+}
+
 // SetTheme sets the element's theme.
 func (element *ScrollContainer) SetTheme (new tomo.Theme) {
 	if new == element.theme.Theme { return }