fix regression in image texture routines. doing nothing on onInvalidate caused the glyphs to be trashed when android was suspended/unsuspended,
calling load was causing a texture reload every glyph (leak), use the invalidate contents method to put the texture in the correct state to load its image into gpu next call (and reuse the texture handle)
Merge branch 'pull_font_fix_android' into pull_font_fix
Merge pull request #1007 from moaiforge/pull_font_fix

Regression in font glyph (imagetexture) invalidation