add fallback character when a character is missing from custom font

[rithik56]

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