Previewly monochrome bitmaps were being created by CreateCompatibleBitmap
because the hdc passed into that call came from CreateCompatibleDC which is
initialized with a single pixel monochrome bitmap.

This makes sure that the flag is set correctly when
surface_allocate_surface is called and client storage is disabled
properly for converted surfaces.

Use plain xrender_blit for mono -> mono copies, masking in combination
with PictOpOver doesn't make sense and doesn't work.

Fixes a Valgrind warning.