main/glsles: return two minor digits when returning SHADING_LANGUAGE_VERSION

For OpenGL ES 3.0 spec, the minor number for SHADING_LANGUAGE_VERSION is always
two digits, matching the OpenGL ES Shading Language Specification release
number. For example, this query might return the string "3.00".

For OpenGL ES 2.0 spec, the version number is either of the form major
number.minor number or major number.minor number.release number, where the
numbers all have one or more digits.

So we return the required format for OpenGL ES 3.0 as it is compatible with
OpenGL ES 2.0 spec.

Signed-off-by: Samuel Iglesias Gonsalvez <siglesias@igalia.com>