[Impeller] disable blending in gaussian intermediate steps. (#51118)

Noticed this while working on the StC debugging. We can disable blending on all intermediate gaussian steps. I don't know how much of an impact this has in practice.

Author: Jonah Williams

impeller/entity/contents/filters/gaussian_blur_filter_contents.cc

@@ -12,9 +12,7 @@
 #include "impeller/entity/contents/content_context.h"
 #include "impeller/entity/texture_fill.frag.h"
 #include "impeller/entity/texture_fill.vert.h"
-#include "impeller/renderer/command.h"
 #include "impeller/renderer/render_pass.h"
-#include "impeller/renderer/texture_mipmap.h"
 #include "impeller/renderer/vertex_buffer_builder.h"
 
 namespace impeller {
@@ -89,6 +87,7 @@ fml::StatusOr<RenderTarget> MakeDownsampleSubpass(
         pass.SetCommandLabel("Gaussian blur downsample");
         auto pipeline_options = OptionsFromPass(pass);
         pipeline_options.primitive_type = PrimitiveType::kTriangleStrip;
+        pipeline_options.blend_mode = BlendMode::kSource;
         pass.SetPipeline(renderer.GetTexturePipeline(pipeline_options));
 
         TextureFillVertexShader::FrameInfo frame_info;
@@ -150,6 +149,7 @@ fml::StatusOr<RenderTarget> MakeBlurSubpass(
 
         ContentContextOptions options = OptionsFromPass(pass);
         options.primitive_type = PrimitiveType::kTriangleStrip;
+        options.blend_mode = BlendMode::kSource;
 
         if (tile_mode == Entity::TileMode::kDecal &&
             !renderer.GetDeviceCapabilities()