Looks like it is on by default. Check the log file.
C:\Users\user_name\AppData\Local\Plex\Logs\Plex.log
2020-05-28T19:25:17.780 [ INFO ] [OpenGL] Current OpenGL ES version 2.0
2020-05-28T19:25:17.780 [ INFO ] [OpenGL] Testing OpenGL ES version 3
...
2020-05-28T19:25:17.780 [ INFO ] [OpenGL] Selected OpenGL ES version 3
...
2020-05-28T19:25:19.339 [ DEBUG ] [mpv] libmpv_render: Detected GLES 3.0.
2020-05-28T19:25:19.339 [ DEBUG ] [mpv] libmpv_render: GL_VENDOR='Google Inc.'
2020-05-28T19:25:19.339 [ DEBUG ] [mpv] libmpv_render: GL_RENDERER='ANGLE (NVIDIA GeForce GTX 750 Ti Direct3D11 vs_5_0 ps_5_0)'
2020-05-28T19:25:19.339 [ DEBUG ] [mpv] libmpv_render: GL_SHADING_LANGUAGE_VERSION='OpenGL ES GLSL ES 3.00 (ANGLE 2.1.0.57ea533f79a7)'
2020-05-28T19:25:19.339 [ DEBUG ] [mpv] libmpv_render: Loaded extension GL_EXT_texture_norm16.
2020-05-28T19:25:19.339 [ DEBUG ] [mpv] libmpv_render: Loaded extension GL_EXT_color_buffer_half_float.
2020-05-28T19:25:19.339 [ DEBUG ] [mpv] libmpv_render: Loaded extension GL_EXT_disjoint_timer_query.
2020-05-28T19:25:19.339 [ DEBUG ] [mpv] libmpv_render: Loaded extension GL_ANGLE_translated_shader_source.
