[FEATURE] Add support for gradient fills for polygons
Increase number of allowed open files on Mac

- Increase from default 256 to 4096-to-8192 range, or OPEN_MAX for process
- From code provided by Larry Biehl, Purdue University, USA, from 'MultiSpec' project
Add expression constructor to QgsFeatureRequest
[Expression] redirect duplicate QString expression() to QString dump()
Merge pull request #923 from nyalldawson/gradient_fill2

[FEATURE] Add support for gradient fills for polygons