Move assigning src in BaseTexture.fromImage. Allows error handling & see...

...ms to've solved a black rectangle texture problem I'd been experiencing

From #918 it seems my original suggestion to move a line worked

Black rectangle problem was happening frequently with http://etg.dek.im/vanilla on any textures at random (I've replaced pixi on etg.dek.im with this pull request's version so the problem should no longer be reproducible there)

Unfortunately fromImage's error handler can't be set before assigning src, so this solution doesn't deal with consistent error detection. Unsure which method is thought best (ie add error handler parameters to fromImage, or have fromImage take a preexisting BaseTexture which has had handlers set already)