main: round floating-point value to nearest integer in glGetSamplerParameteriv()

main: round floating-point value to nearest integer in glGetSamplerParameteriv()

Previously, a cast was done to convert from float to int but there
were rounding errors.

The spec specificies in Data Conversion chapter that Floating-point values are
rounded to the nearest integer.

This patch fixes the following 2 dEQP tests:

dEQP-GLES3.functional.state_query.sampler.sampler_texture_min_lod_getsamplerparameteri
dEQP-GLES3.functional.state_query.sampler.sampler_texture_max_lod_getsamplerparameteri

Signed-off-by: Samuel Iglesias Gonsalvez <siglesias@igalia.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>