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

impeller/entity/contents/gradient_generator.h

@@ -20,6 +20,10 @@ namespace impeller {
 
 class Context;
 
+/// Lower iOS version require alignment to be a multiple of 16, so the alignof
+/// operator will not give correct results for most structs.
+constexpr size_t kSSBOAlignOf = 16u;
+
 /**
  * @brief Create a host visible texture that contains the gradient defined
  * by the provided gradient data.

impeller/entity/contents/linear_gradient_contents.cc

@@ -135,7 +135,7 @@ bool LinearGradientContents::RenderSSBO(const ContentContext& renderer,
 
   gradient_info.colors_length = colors.size();
   auto color_buffer = host_buffer.Emplace(
-      colors.data(), colors.size() * sizeof(StopData), alignof(StopData));
+      colors.data(), colors.size() * sizeof(StopData), kSSBOAlignOf);
 
   VS::FrameInfo frame_info;
   frame_info.mvp = Matrix::MakeOrthographic(pass.GetRenderTargetSize()) *

impeller/entity/contents/radial_gradient_contents.cc

@@ -71,7 +71,7 @@ bool RadialGradientContents::RenderSSBO(const ContentContext& renderer,
 
   gradient_info.colors_length = colors.size();
   auto color_buffer = host_buffer.Emplace(
-      colors.data(), colors.size() * sizeof(StopData), alignof(StopData));
+      colors.data(), colors.size() * sizeof(StopData), kSSBOAlignOf);
 
   VS::FrameInfo frame_info;
   frame_info.mvp = Matrix::MakeOrthographic(pass.GetRenderTargetSize()) *

impeller/entity/contents/sweep_gradient_contents.cc

@@ -77,7 +77,7 @@ bool SweepGradientContents::RenderSSBO(const ContentContext& renderer,
 
   gradient_info.colors_length = colors.size();
   auto color_buffer = host_buffer.Emplace(
-      colors.data(), colors.size() * sizeof(StopData), alignof(StopData));
+      colors.data(), colors.size() * sizeof(StopData), kSSBOAlignOf);
 
   VS::FrameInfo frame_info;
   frame_info.mvp = Matrix::MakeOrthographic(pass.GetRenderTargetSize()) *