add support for lazy-loading tiled images

[chrstphrbrns]

Fixes #164

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 92.68%. Comparing base (1eec9cf) to head (49019ad).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #165      +/-   ##
==========================================
- Coverage   93.67%   92.68%   -0.99%     
==========================================
  Files          13       13              
  Lines        1122     1121       -1     
==========================================
- Hits         1051     1039      -12     
- Misses         71       82      +11     

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