Merge pull request #182 from nwin/master

Deprecation of pixels_mut()…
Merge pull request #185 from nwin/master

some improvements
Fix potential crash caused by channels4()
Implement Index trait for Pixel implementations
Implement IndexMut trait for Pixel implementations
Merge pull request #187 from b52/master

Fix potential crash caused by channels4()
Merge pull request #188 from b52/convenience

Implement Index and IndexMut for Pixel implementations
Add a few optimization hints #189
fixes #186