Fix custommaterial example with OpenGL

The member names in the "uniform block" (which is in practice
mapped to a struct uniform with GLSL) must match between the
vertex and fragment shaders, even if the member is not used in
one of the shaders. This is because OpenGL/GLSL's interface
matching rules, which would cause the shader program fail to
link due to a mismatch in the structs in the two shaders.

Pick-to: 6.1 6.0
Change-Id: I9a928b4b20fdff14530c16a93a52967ce6c328a3
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

examples/quick/scenegraph/custommaterial/shaders/mandelbrot.vert

@@ -8,7 +8,7 @@ layout(location = 0) out vec2 vTexCoord;
 layout(std140, binding = 0) uniform buf {
     mat4 qt_Matrix;
     float qt_Opacity;
-    float scale;
+    float zoom;
     vec2 center;
     int limit;
 } ubuf;