Fix a bug that font size may be smaller than normal after toggling theming.

For example, the following steps will change the system metrics to incorrect values.
1. Open winecfg, and set DPI to 192. Do not restart winecfg.
2. Change the theme to Light. UXTHEME_BackupSystemMetrics() backs up system metrics in 96 DPI
because the current DPI is still 96.
3. Restart winecfg and turn off theming. UXTHEME_RestoreSystemMetrics() restores system metrics in
192 DPI because the current DPI is 192. So system metrics will be scaled to 1/2 of the original size.

Wine-Bug: https://bugs.winehq.org/show_bug.cgi?id=37592Signed-off-by: Zhiyi Zhang <zzhang@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>