Theme Engine: Fix progress bar slider (FS#12823)

Theme Engine: Fix progress bar slider (FS#12823)

Fix the need to resize progress bar when using a slider.
Slider stays in bounds. Progress bar is not resized.
Works well with square sliders that should not overlap
edge of progress bar. also works with rounded sliders.

You can also make progress bar the full width of screen
while using a slider. Before if you would make the
progress bar the full width of the screen and add a slider
the progress bar would look like it was padded on both ends.

This fixes FS#12823

Change-Id: I60345efc5cd0f46286f2591ed032f0d9320d1c3e
Reviewed-on: http://gerrit.rockbox.org/402
Reviewed-by: Hayden Pearce <saint.lascivious@gmail.com>
Tested-by: Hayden Pearce <saint.lascivious@gmail.com>
Reviewed-by: Jonathan Gordon <rockbox@jdgordon.info>