Merge pull request #14912 from davidflanagan/bug939679

Bug 939679: compute the right camera preview size on HD devices r=dmarcos(cherry picked from commit 4245fb93e747d3e3cc9e50ba6fa2f084c9cb682f)
Merge pull request #15085 from davidflanagan/bug939679v1.3

Merge pull request #14912 from davidflanagan/bug939679