Add a --scale-from option

Add a --scale-from option

A typical case for wanting to specify a scale on an output is making your
framebuffer be one size and scaling it to fill an output of a different size.
Instead of making the user calculate the scaling factors to be specified by
--scale, add a new option, --scale-from, that lets the user specify the
framebuffer size directly.  Compute the appropriate transform to achieve the
desired target size.

Signed-off-by: Aaron Plattner <aplattner@nvidia.com>
Reviewed-by: Andy Ritger <aritger@nvidia.com>