Integrate view-map into ordoor

Now we can view scenario maps from the main game interface. We can't
cancel out of a scenario yet, though.

internal/scenario/scenario.go

@@ -0,0 +1,41 @@
+// package play takes a map and turns it into a playable scenario
+package scenario
+
+import (
+	"fmt"
+	"image"
+
+	"code.ur.gs/lupine/ordoor/internal/assetstore"
+)
+
+type Scenario struct {
+	area *assetstore.Map
+
+	tick int
+	turn int
+
+	// All these must be modified by user actions somehow.
+	// TODO: extract into the idea of a viewport passed to Update / Draw somehow?
+	// Or have a separater Drawer for the Scenario?
+	Viewpoint image.Point // Top-left of the screen
+	ZIdx      int         // Currently-viewed Z index
+}
+
+func NewScenario(assets *assetstore.AssetStore, name string) (*Scenario, error) {
+	area, err := assets.Map(name)
+	if err != nil {
+		return nil, err
+	}
+
+	// Eager load sprites. TODO: do we really want to do this?
+	if err := area.LoadSprites(); err != nil {
+		return nil, fmt.Errorf("Eager-loading sprites failed: %v", err)
+	}
+
+	out := &Scenario{
+		area:      area,
+		Viewpoint: image.Pt(0, 3000), // FIXME: haxxx
+	}
+
+	return out, nil
+}