Scenario viewpoint, Z index management, and arrow controls

internal/scenario/scenario.go

@@ -21,6 +21,7 @@ type Scenario struct {
 	// Or have a separater Drawer for the Scenario?
 	Viewpoint image.Point // Top-left of the screen
 	ZIdx      int         // Currently-viewed Z index
+	Zoom      float64     // Zoom level to set
 }
 
 func NewScenario(assets *assetstore.AssetStore, name string) (*Scenario, error) {
@@ -43,6 +44,7 @@ func NewScenario(assets *assetstore.AssetStore, name string) (*Scenario, error)
 		area:      area,
 		specials:  specials,
 		Viewpoint: image.Pt(0, 3000), // FIXME: haxxx
+		Zoom:      1.0,
 	}
 
 	return out, nil