glsl: Respect std430 layout in lower_buffer_access

Respect the std430 rules for determining offset and size of struct
members when using a std430 buffer. std140 rules lead to wrong buffer
offsets in that case.

Fixes my test case attached in Bugzilla. No piglit changes.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=104492


Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
(cherry picked from commit 7e025def)