change missile ready texture swapping to use modulate instead

[thoraxe]

Examples: my_texture_rect.modulate = Color(0.8, 0, 0) my_texture_rect.modulate = Color.red

[thoraxe]

https://godotengine.org/qa/74313/is-it-better-to-use-a-sprite-or-a-texturerect-for-a-background