Adjust maximum texture size to be configurable

[adamwulf]

The max texture size is currently set to be the portrait bounds of the iPad, but that means if a jot view is made in landscape mode, then exported images will be cut off at the portrait width. (see #9)

The workaround is in PortraitBounds.m

// if (b.size.width > b.size.height) {
// CGFloat t = b.size.height;
// b.size.height = b.size.width;
// b.size.width = t;
// }

But a better fix should be added to the texture cache.