Add ability to specify a custom transformation to apply to the current Bitmap

[mbrubin56]

• Add a new interface, Transform, that clients can implement to apply a custom transformation to the GIF's current Bitmap.
• Reimplemented the rounded corners implementation as an implementation of Transform.

[codecov-io]

Current coverage is 3.91%

Merging #259 into master will decrease coverage by -0.06% as of 7c593bd

@@            master   #259   diff @@
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  Files           22     23     +1
  Stmts         1181   1200    +19
  Branches       170    173     +3
  Methods          0      0       
=====================================
  Hit             47     47       
  Partial          5      5       
- Missed        1129   1148    +19

Review entire Coverage Diff as of 7c593bd

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[koral--]

Thanks, apart from comments above LGTM.

[mbrubin56]

Thanks for the comments! I agree with them (sorry I mistranslated that corner radius from a float to an int). Everything should be addressed with ac83411.

[mbrubin56]

Oh, and I should also take the time to thank you for the lovely library.

[koral--]

Merged into dev branch with minor tweaks. This PR will be closed automatically when commit gets merged into the master.