revert oninvalidate override on image texture

By not freeing the texture handle on invalidate, we break fonts on android app resume.
This fixes https://github.com/moai/moai-dev/issues/838

These lines were put in as a fix for texture being realloced for each glyph added. which in turn was caused by
https://github.com/patrickmeehan/moai-dev/commit/96ba90a61897bf7f15bbb45b3547db5748d06e9f
where the line page->mImageTexture->Invalidate (); was added in an attempt to refresh the image texture on glyph addition. I am guessing that is calling the wrong invalidate, we probably just want to update the texture data not blow it away.