Purge sRGB chunks from PNGs in examples.

Subjects each *.png file that matched grep -law "sRGB" to:
pngcrush -ow -brute -rem allb -reduce

Change-Id: I2b8810829f1b91033539229a6d4b3e505b8730ea
Reviewed-by: Topi Reiniö <topi.reinio@theqtcompany.com>