Linux dependencies: Build libpng16 for freetype so can render colored emoji

[misl6]

Maintainer merge checklist

• Title is descriptive/clear for inclusion in release notes.
• Applied a Component: xxx label.
• Applied the api-deprecation or api-break label.
• Applied the release-highlight label to be highlighted in release notes.
• Added to the milestone version it was merged into.
• Unittests are included in PR.
• Properly documented, including versionadded, versionchanged as needed.

freetype (a dependency of SDL_ttf) should be built with libpng support, otherwise colored emoji which make use of PNG (like the default Noto Color Emoji provided on Ubuntu).

FYI: For Noto Color Emoji, only https://github.com/googlefonts/noto-emoji/blob/main/fonts/NotoColorEmoji_WindowsCompatible.ttf and the NotoColorEmoji.ttf available on /usr/share/fonts/truetype/noto/NotoColorEmoji.ttf seem to render correctly.

Linked issue (do not close as support on macOS -and likely other platforms- is missing): #7565

✅ Tested locally
✅ Tested with wheel (manylinux_x86_64) artifact on Ubuntu 22.04 LTS with bundled NotoColorEmoji font
✅ Tested with wheel (linux_armv7l) artifact on RPi 4 Raspberry OS 32 bit (bullseye) with NotoColorEmoji_WindowsCompatible.ttf font (link above)
⚠️ Unfortunately we do not handle (and maybe SDL does not) an error when rendering is not possible, so we can't add a test ATM.

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(100)
        font_name: "NotoColorEmoji"
"""
)


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


Testapp().run()

Output: