[Impeller] allow fixup UBO to have different offsets.

impeller/compiler/reflector.cc

@@ -376,6 +376,8 @@ std::shared_ptr<RuntimeStageData::Shader> Reflector::GenerateRuntimeStageData()
 
     const auto& ubo = ubos[0];
 
+    size_t binding =
+        compiler_->get_decoration(ubo.id, spv::Decoration::DecorationBinding);
     auto members = ReadStructMembers(ubo.type_id);
     std::vector<uint8_t> struct_layout;
     size_t float_count = 0;
@@ -410,9 +412,8 @@ std::shared_ptr<RuntimeStageData::Shader> Reflector::GenerateRuntimeStageData()
     }
     data->uniforms.emplace_back(UniformDescription{
         .name = ubo.name,
-        .location = 64,  // Magic constant that must match the descriptor set
-                         // location for fragment programs.
-        .binding = 64,
+        .location = binding,
+        .binding = binding,
         .type = spirv_cross::SPIRType::Struct,
         .struct_layout = std::move(struct_layout),
         .struct_float_count = float_count,

impeller/runtime_stage/runtime_stage.cc

@@ -89,13 +89,22 @@ RuntimeStage::RuntimeStage(const fb::RuntimeStage* runtime_stage,
   entrypoint_ = runtime_stage->entrypoint()->str();
 
   auto* uniforms = runtime_stage->uniforms();
+
+  // Note: image bindings are screwy and will always have the same offset.
+  // track the binding of the UBO to determine where the image bindings go.
+  // This is only guaranteed to give us the correct bindings if there is a
+  // single sampler2D.
+  std::optional<size_t> ubo_id;
   if (uniforms) {
     for (auto i = uniforms->begin(), end = uniforms->end(); i != end; i++) {
       RuntimeUniformDescription desc;
       desc.name = i->name()->str();
       desc.location = i->location();
       desc.binding = i->binding();
       desc.type = ToType(i->type());
+      if (desc.type == kStruct) {
+        ubo_id = desc.location;
+      }
       desc.dimensions = RuntimeUniformDimensions{
           static_cast<size_t>(i->rows()), static_cast<size_t>(i->columns())};
       desc.bit_width = i->bit_width();
@@ -105,7 +114,6 @@ RuntimeStage::RuntimeStage(const fb::RuntimeStage* runtime_stage,
           desc.struct_layout.push_back(static_cast<uint8_t>(byte_type));
         }
       }
-      desc.binding = i->binding();
       desc.struct_float_count = i->struct_float_count();
       uniforms_.push_back(std::move(desc));
     }
@@ -117,6 +125,20 @@ RuntimeStage::RuntimeStage(const fb::RuntimeStage* runtime_stage,
       [payload = payload_](auto, auto) {}  //
   );
 
+  size_t binding = 64;
+  if (ubo_id.has_value() && ubo_id.value() == binding) {
+    binding++;
+  }
+  for (auto& uniform : uniforms_) {
+    if (uniform.type == kSampledImage) {
+      uniform.binding = binding;
+      binding++;
+      if (ubo_id.has_value() && ubo_id.value() == binding) {
+        binding++;
+      }
+    }
+  }
+
   for (const auto& uniform : GetUniforms()) {
     if (uniform.type == kStruct) {
       descriptor_set_layouts_.push_back(DescriptorSetLayout{
@@ -132,7 +154,6 @@ RuntimeStage::RuntimeStage(const fb::RuntimeStage* runtime_stage,
       });
     }
   }
-
   is_valid_ = true;
 }