fix stripping for wrapped text
Merge pull request #2677 from kived/textwrap-strip

fix stripping for wrapped text
doc: add a Common Pitfalls section to RelativeLayout
add images
fix export_to_png with img_sdl2
Merge pull request #2679 from kived/doc-relativelayout-pitfalls

doc: add a Common Pitfalls section to RelativeLayout
Optimize doc images with trimage
Merge pull request #2681 from kivy/doc_image_optim

Optimize doc images with trimage
Merge pull request #2680 from kived/sdl2-exportpng

fix export_to_png with img_sdl2
sdl2/image: remove the bad +36, and use char* cast to get the string, not a pointer to the python object.