[client] egl: correct assign to gl_Position in basic.vert

gl_Position is expected to be using homogeneous coordinates, which requires
w to be a coordinate scale factor, usually 1.0. z should also be set in order
for depth to be well-defined. Therefore, we should set gl_Position.zw to
vec2(0.0, 1.0).

client/renderers/EGL/shader/basic.vert

@@ -9,6 +9,6 @@ out vec2 iFragCoord;
 
 void main()
 {
-  gl_Position.xy = uVertex;
-  iFragCoord     = uUV;
+  gl_Position = vec4(uVertex, 0.0, 1.0);
+  iFragCoord  = uUV;
 }