android: display: use OpenGL to display subpictures

Replace the old native_window_api_t code with the new vlc_gl_sub_renderer() API in order to:

• Use common code (more code removed than added)
• Improve upload performances

Commits !3346 (e00b58d3) and !3346 (9ff2d367) are separated to ease the review process (otherwise, the diff is unreadable). Maybe you want to squash them before merge ?

Benchmark result when using a low end android armv7 box on 4K display:

• Complex subtitles via https://streams.videolan.org/samples/fate-suite/sub/
  • Before: Around 80-120ms for 1 upload
  • Now: 8-10ms for 1 upload
• EIA-608 subtitles
  • Before: too slow resulting on mass frame drops (lasting even after subtitles are cleared)
  • Now: smooth.

Depends on libvlcjni!50 (merged)