[Impeller] align SSBOs to 16 bytes to fix iOS shader validation issue. (#39654)

* [Impeller] align SSBOs to 16 bytes to fix iOS shader validation issue.

* use DefaultUniformAlignment

Author: Jonah Williams

impeller/entity/contents/linear_gradient_contents.cc

@@ -134,8 +134,9 @@ bool LinearGradientContents::RenderSSBO(const ContentContext& renderer,
   auto colors = CreateGradientColors(colors_, stops_);
 
   gradient_info.colors_length = colors.size();
-  auto color_buffer = host_buffer.Emplace(
-      colors.data(), colors.size() * sizeof(StopData), alignof(StopData));
+  auto color_buffer =
+      host_buffer.Emplace(colors.data(), colors.size() * sizeof(StopData),
+                          DefaultUniformAlignment());
 
   VS::FrameInfo frame_info;
   frame_info.mvp = Matrix::MakeOrthographic(pass.GetRenderTargetSize()) *

impeller/entity/contents/radial_gradient_contents.cc

@@ -70,8 +70,9 @@ bool RadialGradientContents::RenderSSBO(const ContentContext& renderer,
   auto colors = CreateGradientColors(colors_, stops_);
 
   gradient_info.colors_length = colors.size();
-  auto color_buffer = host_buffer.Emplace(
-      colors.data(), colors.size() * sizeof(StopData), alignof(StopData));
+  auto color_buffer =
+      host_buffer.Emplace(colors.data(), colors.size() * sizeof(StopData),
+                          DefaultUniformAlignment());
 
   VS::FrameInfo frame_info;
   frame_info.mvp = Matrix::MakeOrthographic(pass.GetRenderTargetSize()) *

impeller/entity/contents/sweep_gradient_contents.cc

@@ -76,8 +76,9 @@ bool SweepGradientContents::RenderSSBO(const ContentContext& renderer,
   auto colors = CreateGradientColors(colors_, stops_);
 
   gradient_info.colors_length = colors.size();
-  auto color_buffer = host_buffer.Emplace(
-      colors.data(), colors.size() * sizeof(StopData), alignof(StopData));
+  auto color_buffer =
+      host_buffer.Emplace(colors.data(), colors.size() * sizeof(StopData),
+                          DefaultUniformAlignment());
 
   VS::FrameInfo frame_info;
   frame_info.mvp = Matrix::MakeOrthographic(pass.GetRenderTargetSize()) *