Enable libpng features used by cairo

This enables the bKGD (background color) feature and time conversion
utilities which are used by cairo. Specifically this flips the
following defines on:

PNG_CONVERT_tIME_SUPPORTED
PNG_bKGD_SUPPORTED
PNG_tIME_SUPPORTED

Change-Id: I59396acd54c760d2f28b3159973993b3676c5c4c
1 file changed