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/shaders/egl.frag

@@ -0,0 +1,21 @@
+#version 300 es
+#extension GL_OES_EGL_image_external : require
+precision mediump float;
+out vec4 FragColor;
+
+in vec2 vTextCoord;
+
+uniform mat3 yuvmat;
+uniform vec3 offset;
+uniform samplerExternalOES plane1;
+uniform samplerExternalOES plane2;
+
+void main() {
+	vec3 YCbCr = vec3(
+		texture2D(plane1, vTextCoord)[0],
+		texture2D(plane2, vTextCoord).xy
+	);
+
+	YCbCr -= offset;
+	FragColor = vec4(clamp(yuvmat * YCbCr, 0.0, 1.0), 1.0f);
+}

app/shaders/egl.vert

@@ -0,0 +1,10 @@
+#version 300 es
+
+layout (location = 0) in vec2 aPosition; // 2D: X,Y
+layout (location = 1) in vec2 aTexCoord;
+out vec2 vTextCoord;
+
+void main() {
+	vTextCoord = aTexCoord;
+	gl_Position = vec4(aPosition, 0, 1);
+}