[feat]IBA:demosaic add X-Trans demosaicing

[antond-weta]

Description

Implements X-Trans pattern demosaicing using a simple bilinear algorithm. Adds the Xtrans pattern layout to the ImageBuf attributes in the raw reader plugin.

Tests

I have implemented unittests in imagebufalgo_test.cpp which cover all permutations of the Bayer and X-Trans layouts, as the X-Trans pattern is a bit too complicated to generate using oiiotool.

Checklist:

• I have read the contribution guidelines.
• I have updated the documentation, if applicable. (Check if there is no
  need to update the documentation, for example if this is a bug fix that
  doesn't change the API.)
• I have ensured that the change is tested somewhere in the testsuite
  (adding new test cases if necessary).
• If I added or modified a C++ API call, I have also amended the
  corresponding Python bindings (and if altering ImageBufAlgo functions, also
  exposed the new functionality as oiiotool options).
• My code follows the prevailing code style of this project. If I haven't
  already run clang-format before submitting, I definitely will look at the CI
  test that runs clang-format and fix anything that it highlights as being
  nonconforming.