loader: reset FONT_MANUAL flag on resolution change
The FONT_MANUAL flag is set on font when font is set by user (in script or command line) to preserve font setting. However, on resolution change, we want to have font auto selection fully operational because user selected font may not be usable - like on case when we set to bios text mode, we can only have font with glyph size 8x16.
in bios mode used both methods to select font (loadfont command and set screen-font) other than 8x16, then switched framebuffer off to get vga text mode. Observerd the text mode has usable font.
Also did change framebuffer resolution with framebuffer set and observed the font change.
Note: we could preserve the user selected font when only the depth is changed (only possible in BIOS mode, uefi only has depth 32), but I do not think it is worth the effort.