loader: gfx_fb_drawrect should use GfxFbBltVideoFill

The gfx_fb_drawrect() is drawing rectangle by pixels, this can be very
slow on some systems. Use Blt() video fill primitive instead.

Testing done: Tested on mac mini 2012 where the issue was revealed

(cherry picked from commit 5365af662c78d7bded3bf83c7271d6bff17229a9)

Reviewed by: yuripv