Add support for libpng (PNG images)

Packages describing “png” as local USE flag

Package“png” Flag Description
app-emulation/qemuEnable png image support for the VNC console server
app-misc/lcdprocEnable support for media-libs/libpng in the glcd driver.
app-text/mathtexGenerate png images by default instead of gif
dev-ruby/barbyAdd support for writing PNG images using dev-ruby/chunky_png.
media-gfx/qrencodeEnable PNG output support in qrencode tool
sci-electronics/pcbPNG graphics export

All packages providing a “png” USE flag (117)