opengl: gl_util: fix -Wsign-compare warning

    ../../test/../modules/video_output/opengl/gl_util.c: In function ‘LogProgramErrors’:
    ../../test/../modules/video_output/opengl/gl_util.c:121:23: warning: comparison of integer expressions of different signedness: ‘size_t’ {aka ‘long unsigned int’} and ‘GLsizei’ {aka ‘int’} [-Wsign-compare]
      121 |     for (size_t i=0; i<shader_count; ++i)
          |                       ^