Add uv offsets to autogenerated 2D uv coordinates.

The original render_gl3.c code does the following:

```
mjr_setf4(plane, 0.5*scl[0], 0, 0, -0.5);
glTexGenfv(GL_S, GL_OBJECT_PLANE, plane);
mjr_setf4(plane, 0, -0.5*scl[1], 0, -0.5);
glTexGenfv(GL_T, GL_OBJECT_PLANE, plane);
```

The 4th component of the GL_OBJECT_PLANE function effectively
describes a UV-offset for the generates uvs. In our filament
implementation, we had overlooked this 4th component.

PiperOrigin-RevId: 863181644
Change-Id: I60688117ea2019257da683cdda0c7d389dd5af1e

src/experimental/filament/assets/phong_2d.mat

@@ -23,6 +23,7 @@ material {
         { type : float, name : GlossinessFactor },
         { type : float, name : EmissiveFactor },
         { type : float3, name : UvScale },
+        { type : float3, name : UvOffset },
         { type : sampler2d, name : BaseColor }
     ],
     variables : [
@@ -38,7 +39,7 @@ vertex {
 
 fragment {
   void material(inout MaterialInputs material) {
-    vec2 uv = variable_vertex_pos.xy * materialParams.UvScale.xy;
+    vec2 uv = variable_vertex_pos.xy * materialParams.UvScale.xy + materialParams.UvOffset.xy;
 
     prepareMaterial(material);
     material.baseColor     = materialParams.BaseColorFactor;

src/experimental/filament/assets/phong_2d_fade.mat

@@ -24,6 +24,7 @@ material {
         { type : float, name : GlossinessFactor },
         { type : float, name : EmissiveFactor },
         { type : float3, name : UvScale },
+        { type : float3, name : UvOffset },
         { type : sampler2d, name : BaseColor }
     ],
     variables : [
@@ -39,7 +40,7 @@ vertex {
 
 fragment {
   void material(inout MaterialInputs material) {
-    vec2 uv = variable_vertex_pos.xy * materialParams.UvScale.xy;
+    vec2 uv = variable_vertex_pos.xy * materialParams.UvScale.xy + materialParams.UvOffset.xy;
 
     prepareMaterial(material);
     material.baseColor     = materialParams.BaseColorFactor;