Minor refactor of built-in QSGVideoNodes.

- Load shader source from resource files.
- Correctly report material types: each material can use different
  shaders depending on the video pixel format but it was reporting
  a unique material type. This was causing the node to keep using
  the same shader even if its pixel format changed.

Change-Id: Ib903ecd6e7dd1dd56d7cefe255ab7049933df17d
Reviewed-by: Gunnar Sletta <gunnar@sletta.org>

src/qtmultimediaquicktools/shaders/rgbvideo.vert

@@ -0,0 +1,9 @@
+uniform highp mat4 qt_Matrix;
+attribute highp vec4 qt_VertexPosition;
+attribute highp vec2 qt_VertexTexCoord;
+varying highp vec2 qt_TexCoord;
+
+void main() {
+    qt_TexCoord = qt_VertexTexCoord;
+    gl_Position = qt_Matrix * qt_VertexPosition;
+}