don't decode twice and encode unnecessarily
don't decode twice and encode unnecessarily

the watermark filter was actually decoding the watermark image 2x
and was also encoding it 1x without a real need to do so.

This also follows the change done in #930 now.

Test:

http://localhost:8888/unsafe/filters:watermark(https://somewhere.com/image.jpg,-10,-10,20)/https://somewhereelse.com/another_image.jpg

Before the change the following engine calls were made:

engine.create_image() # main image decode
engine.create_image() # watermark decode 1st
engine.read()         # watermark encode => storage
engine.create_image() # watermark decode 2nd
engine.read()         # final image generation

After the change:

engine.create_image() # main image decode
engine.create_image() # watermark decode
engine.read()         # final image generation

marcelometal/thumbor,kkopachev/thumbor,thumbor/thumbor,okor/thumbor,thumbor/thumbor,gi11es/thumbor,marcelometal/thumbor,thumbor/thumbor,thumbor/thumbor,scorphus/thumbor,gi11es/thumbor,scorphus/thumbor,voxmedia/thumbor,okor/thumbor,scorphus/thumbor,voxmedia/thumbor,marcelometal/thumbor,kkopachev/thumbor,kkopachev/thumbor,kkopachev/thumbor,okor/thumbor,gi11es/thumbor,voxmedia/thumbor,scorphus/thumbor