Make TIFF strip size configurable

[kmilos]

Fixes #5740

Changes proposed in this pull request:

• Add a way to override the strip size by redefining PIL.TiffImagePlugin.STRIP_SIZE (in bytes)
• Do not round up rows_per_strip, requested strip size should not be exceeded

[radarhere]

I created kmilos#3 to add a test for this.

[kmilos]

Thanks for fixing up the PR!

[hugovk]

Thanks all!