gl-renderer: Implement & use check_extension

gl-renderer: Implement & use check_extension

Using strstr(3) for checking for extensions is an error-prone mechanism
as extension names can be prefixes of other extension names (see
https://www.opengl.org/registry/doc/rules.html#using).

This change implements the check_extension() function to properly check
for an extension and replaces all usages of strstr(3).

Signed-off-by: Miguel A Vico Moya <mvicomoya@nvidia.com>
Reviewed-by: Andy Ritger <aritger@nvidia.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
[Pekka: move 'bool' to the same line with 'static']
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>