bufferpool: Fix the buffer size reset code

The offset in gst_buffer_resize() is additive. So to move back the
offset to zero, we need to pass the opposite of the current offset. This
was raised through the related unit test failingon 32bit as on 64bit
the alignment padding was enough to hide the issue. The test was
modified to also fail on 64bit. This patch will remove spurious
assertions like:

  assertion 'bufmax >= bufoffs + offset + size' failed

Fixes #316
22 jobs for bufferpool-buffer-reset in 45 minutes and 56 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Preparation
passed #392840
gst indent

00:00:49

passed #392839
manifest

00:00:28

 
  Build
passed #392846
build cerbero android universal

00:25:41

passed #392847
build cerbero cross win32

00:13:48

passed #392848
build cerbero cross win64

00:14:06

passed #392845
build cerbero fedora x86_64

00:14:02

manual #392850
gst-ios-12.2 allowed to fail manual
build cerbero ios universal
passed #392849
gst-macos-10.14
build cerbero macos x86_64

00:06:43

passed #392841
build fedora x86_64

00:15:25

passed #392842
build nodebug fedora x86_64

00:07:25

passed #392843
build static fedora x86_64

00:22:06

passed #392844
build static nodebug fedora x86_64

00:17:06

 
  Test
passed #392851
check fedora

00:06:08

passed #392852
integration testsuites fedora

00:13:53

passed #392857
valgrind bad

00:09:22

passed #392854
valgrind base

00:11:01

passed #392853
valgrind core

00:10:54

passed #392858
valgrind ges

00:05:24

passed #392855
valgrind good

00:09:40

passed #392856
valgrind ugly

00:03:52

 
  Integrate
passed #392859
android universal examples

00:05:30

passed #392860
documentation

00:04:30