app/internal/gpu: default to color material

Before this change the default material were a texture, which leads
to a crash when no texture is current.

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

app/internal/gpu/gpu.go

@@ -212,8 +212,8 @@ const (
 )
 
 const (
-	materialTexture materialType = iota
+	materialColor materialType = iota
-	materialColor
+	materialTexture
 )
 
 var (