add fallback character when a character is missing from custom font #13070
      
        
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This closes the good first issue #12975
Added an option to show the fallback character when a character from custom font is missing.
Added a new a field in GlyphManager class, enableFallbackGlyph which if set to true, will replace the missing characters with ? of that font stack
If we have enabled this option, then if no character of that font stack is present between unicode id 0 and 255, then we would load 0-255.pbf file of that font stack so that we retrieve the glyph of ? of that fontstack and show that as fallback if any character is missing