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
21 jobs for backport-mr-193-into-1.16-bufferpool-buffer-reset in 90 minutes and 45 seconds (queued for 20 seconds)
Status Job ID Name Coverage
  Preparation
passed #488302
gst indent

00:00:39

passed #488301
manifest

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  Build
passed #488308
build cerbero android universal

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passed #488309
build cerbero cross win32

00:07:37

passed #488310
build cerbero cross win64

00:04:56

passed #488307
build cerbero fedora x86_64

00:07:35

passed #488312
gst-ios-12.2
build cerbero ios universal

00:34:56

passed #488311
gst-macos-10.14
build cerbero macos x86_64

00:09:36

passed #488303
build fedora x86_64

00:05:49

passed #488304
build nodebug fedora x86_64

00:05:15

passed #488305
build static fedora x86_64

00:12:46

passed #488306
build static nodebug fedora x86_64

00:10:42

 
  Test
passed #488313
check fedora

00:03:20

passed #488314
integration testsuites fedora

00:12:59

passed #488319
valgrind bad

00:12:09

passed #488316
valgrind base

00:08:00

passed #488315
valgrind core

00:10:27

passed #488320
valgrind ges

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passed #488317
valgrind good

00:15:19

passed #488318
valgrind ugly

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  Integrate
passed #488321
android universal examples

00:08:37