glsl: explicitly zero out padding to gl_shader_variable bitfield

glsl: explicitly zero out padding to gl_shader_variable bitfield

Otherwise, the padding bits remain undefined, which leads to valgrind
errors when storing the gl_shader_variable in the disk cache.

v2: use rzalloc instead of an explicit padding member variable

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>