Update `libpng` recipe and build `SDL2_ttf` vendored `freetype` with png support, so can render colored emoji

[misl6]

See: https://github.com/kivy/kivy/issues/7565

Apple Color Emoji and other system fonts are not accessible on iOS, as are located outside of the app sandbox, so the user will need to use an alternative font ATM.

PS: Accessing the system fonts is feasible, via CGFont methods (by reconstructing a .ttf file), and we likely want to introduce this feature on kivy/kivy.

Demo:

from kivy.app import App
from kivy.lang import Builder
from kivy.uix.boxlayout import BoxLayout

class UI(BoxLayout):
    pass

Builder.load_string(
    """
<UI>:
    Label:
        text: "😀 🎉 📷 👕 🐞"
        font_size: dp(80)
        font_name: "./NotoColorEmoji_WindowsCompatible.ttf"
"""
)

class Testapp(App):
    def build(self):
        return UI()

Testapp().run()

Output: