little changes to get it work in armv7

i did dome little changes to your code to get i t work in arm 7 arch. in a OFish style, i commented out a glPushAttrib()  here:  https://github.com/kalwalt/ofxThreadedImage/compare/armadillu:master...master#diff-a2347236d59351c521150a20bd4a789aL171 in order to get it work. i don't know how to transform in a crossplatform way or if it is absolutely needed.
Thank's
Walter