Add a setting for the resolution of the exported image

[Stefano Verzegnassi]

Currently all the images processed by InstantFX have a maximum resolution of 2048x2048 px.

We should allow to set this value, in order to optimize performance on low-end device or to allow an higher quality on high-end devices.

Ideally, the choice for the maximum resolution should be between one of the following value:
- 960 px
- 1280 px
- 2048 px
- 3072 px
- 4096 px (To be tested, not sure if Aquaris E5 can support such size)