INT 62 u - FGDRIVER v3.03 - "FG_PCXMODE" - GET OPTIMAL VIDEO MODE FOR DISPLAY
        AX = 0074h
        ES:BX -> PCX header (see AX=0073h)
Return: AX = optimal video mode for PCX file
            FFFEh not a valid PCX header
            FFFFh unable to determine compatible video mode
Note:   For FGDRIVER v1.10, this function was "FG_SCRLOCK" (see AX=0088h);
           "FG_PCXMODE" did not exist yet
SeeAlso: AX=0073h,AX=009Ah

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