common: add mp_snprintf_append() utility function
vo_vdpau: let vdpau scale anamorphic screenshots

In theory, returning the screenshot with original pixel aspect would
allow avoiding scaling them with image formats that support non-square
pixels, but in practice this isn't used anyway (nothing seems to
understand e.g. jpeg aspect ratio tags).
video: remove redundant mp_image display_w/_h members

Commit 5e4e248 added a mp_image_params field to mp_image, and moved many
parameters to that struct. display_w/h was left redundant with
mp_image_params.d_w/d_h. These fields were supposed to be always in
sync, but it seems some code forgot to do this correctly, such as
vf_fix_img_params() or mp_image_copy_attributes(). This led to the
problem in github issue #756, because display_w/_h could become
incorrect.

It turns out that most code didn't use the old fields anyway. Just
remove them. Note that mp_image_params.d_w/d_h are supposed to be always
valid, so the additional checks for 0 shouldn't be needed. Remove these
checks as well.

Fixes #756.