FS-7500: libpng 1.6.0 has much simpler APIs, also add a new switch_img_patch_png...

FS-7500: libpng 1.6.0 has much simpler APIs, also add a new switch_img_patch_png to possible patch a transparent png to an img

While this is not optimal, we should cache the png:
1) cache the whole buffer, with comes with RGBARGBA pixel formats
2) Allow switch_image_t to be other formats e.g. VPX_IMG_FMT_ARGB, VPX_IMG_FMT_ARGB_LE, or VPX_IMG_FMT_444A
   those can have alpha channels so we can check the alpha channel before we patch

Note all PNG are created equel, or maybe a bug in libpng since for some PNG files with alpha the returned buffer
     not seems like RGBARGBA... while docs says it should default be RGBA