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;

src/experimental/filament/filament/drawable.cc

@@ -484,8 +484,15 @@ void Drawable::UpdateMaterial(const mjvGeom& geom, bool use_segid_color) {
         params.uv_scale.y = 2.0f * plane_scale / tile_size_y;
       }
 
+      // We want to do the equivalent of:
+      //   mjr_setf4(splane, 0.5 * scl.x,  0, 0, -0.5);
+      //   mjr_setf4(tplane, 0, -0.5 * scl.y, 0, -0.5);
+      //   glTexGenfv(GL_S, GL_OBJECT_PLANE, splane);
+      //   glTexGenfv(GL_T, GL_OBJECT_PLANE, tplane);
       params.uv_scale.x = 0.5f * params.uv_scale.x;
-      params.uv_scale.y = 0.5f * params.uv_scale.y;
+      params.uv_scale.y = -0.5f * params.uv_scale.y;
+      params.uv_offset.x = -0.5f;
+      params.uv_offset.y = -0.5f;
     } else {
       // For cube maps, if `tex_uniform` is true, then scale the texture so that
       // it covers a 1x1 area of world space rather than the area of the object.

src/experimental/filament/filament/material.cc

@@ -110,6 +110,9 @@ void Material::UpdateMaterialInstances() {
   if (material->hasParameter("UvScale")) {
     instance->setParameter("UvScale", params_.uv_scale);
   }
+  if (material->hasParameter("UvOffset")) {
+    instance->setParameter("UvOffset", params_.uv_offset);
+  }
 
   if (instances_[DrawMode::kSegmentation]) {
     instances_[DrawMode::kSegmentation]->setParameter(

src/experimental/filament/filament/material.h

@@ -54,6 +54,7 @@ class Material {
     filament::math::float4 segmentation_color = {1, 1, 1, 1};
     filament::math::float2 tex_repeat = {1, 1};
     filament::math::float3 uv_scale = {1, 1, 1};
+    filament::math::float3 uv_offset = {0, 0, 0};
     float specular = -1.0f;
     float glossiness = -1.0f;
     float metallic = -1.0f;