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As the number of digits in the CPU usage labels vary, their width was
varying too. When the window was narrow enough to be around the
natural width of the graphs, this could cause the width of all the
graphs on the Resources tab to resize.

Set a desired width of 6 characters on these labels. This is hopefully
enough for all values ("100.0%" is probably the longest), preventing
the size changing with the value, but it may vary depending on font.

Fixes #149, assuming the other regularly changing labels are not the
limiting factor in the width of the Resources tab.

(cherry picked from commit 4b7d37bc)

(cherry picked from commit 35f88a56)

I couldn't figure out why the math was wrong, and line 325 `cairo_rel_line_to (cr, x_offset * 2, 0);` is a hack,
but now it is finally painting in the bottom right corner properly.

I struggled with this for two hours.

Also fixed a bug with the stacked view mode, where it didn't draw the right-most part correctly.
But previously you couldn't see that because it always was outside the visible part.

It was drawn to far to the right and took several seconds to wander into the visible area.