tuttleHost: OfxhProperty::getStringValue() now represents all the values

- getStringValue() now returns a string representing all the values
- getStringValueAt(int) to get a value at a specific index
- getStringValues() to get an array of values
plugins: fix renderscale on generators

Fix #352
Fix #356
tuttlePlugin: initialize the renderWindow instead of the imageBounds
plugins: use renderscale in fetchImage