texture: fix invalid color conversion for texture when introduced icolorfmt (need double check with SMAA).

+ fixes for BRGA -> RGBA conversion (the implemented conversion was about ARGB -> RGBA).
+ fixes crash when Py_Decref the returned buffer. I do remember an issue with it before, but in this case, the buffer returned was having no reference at all = segfault.

Closes #2627
doc: syntax
dictadapter: fix unit tests related to it. ref #1972
tests: add the possibility to run tests without internet (use NONETWORK=1 make test)
fix #2732 use list() copy
fix example in docstring for kivy.cache module
fix example in docstring for Cache
remove wrongful note, do_default_tab still defaults to True
Merge pull request #2737 from terrex/master

fix example in docstring for kivy.cache module