Improve Quicktime Metadata Atom handling

[nagix]

Problem

NullPointerException occurs on some MOV file inputs. The cause is likely to be an undefined metadata tag in the Quicktime Metadata Atom.

Solution

• Add null check for tag ID lookup
• Add com.apple.quicktime.pixeldensity tag
• The current implementation interprets a value as a string using the default encoding, but it can be other data types such as integer, array, etc. So, add the code to read the data type field and set it (Reference)
• Add tag constants for QuickTimeMetadataDirectory
• Add tag descriptions for some tags

[drewnoakes]

👍 Looks good to me.

@payton is more familiar with the code so I'll let him review/merge.

[payton]

Looks good!