MdeModulePkg/HiiDatabase: Preinit background when render string to screen

MdeModulePkg/HiiDatabase: Preinit background when render string to screen

When render string to screen, for proportional fonts,
the background color may not set to the whole BltBuffer.
And this will cause incorrect display.
Now initialize the background color to fix this issue.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Dandan Bi <dandan.bi@intel.com>
Reviewed-by: Eric Dong <eric.dong@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>