macOS Big Sur support

update macOS build instructions, travis no longer exists

Don't use const on image2d_t

Run GitHub workflow build on pull requests

User guide from HePTaset

added missing QPainterPath includes

Pushing a branch and creating a pull request would run all the builds twice with the previous formulation.

Trying the yml from here: https://github.community/t/how-to-trigger-an-action-on-push-or-pull-request-but-not-both/16662/3

fix typo on preset include

First Batch of Presets (ZCam S1, GoPro Fusion, Yi360)

The type qualifiers const, restrict and volatile as defined by the
C99 specification are supported. These qualifiers cannot be used with
image2d_t or image3d_t type. Types other than pointer types shall not
use the restrict qualifier.

See discussion in https://github.com/stitchEm/stitchEm/issues/100