Introduce a new FFMPEG frontend renderer: EGLRenderer

Right now this renderer works on X11 & Wayland with VAAPI as a backend.

Some rendering latency benchmarks on my `i7-10510U` (with
`intel-media-driver` 20.1.1 which cause a *huge* regression with the
SDL_Renderer):
|             | X11    | Wayland |
| Before      | 6.78ms | 22.50ms |
| EGLRenderer | 0.76ms | 00.77ms |

Signed-off-by: Antoine Damhet <antoine.damhet@lse.epita.fr>

app/app.pro

@@ -90,6 +90,10 @@ unix:!macx {
             CONFIG += libva
         }
 
+        packagesExist(egl) {
+            CONFIG += egl
+        }
+
         packagesExist(vdpau) {
             CONFIG += libvdpau
         }
@@ -264,6 +268,15 @@ libdrm {
     SOURCES += streaming/video/ffmpeg-renderers/drm.cpp
     HEADERS += streaming/video/ffmpeg-renderers/drm.h
 }
+egl {
+    message(EGL renderer selected)
+
+    DEFINES += HAVE_EGL
+    SOURCES += \
+        streaming/video/ffmpeg-renderers/eglvid.cpp \
+        streaming/video/ffmpeg-renderers/egl_extensions.cpp
+    HEADERS += streaming/video/ffmpeg-renderers/eglvid.h
+}
 config_SL {
     message(Steam Link build configuration selected)