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

This reverts commit 742c780.

Author: auto-submit[bot]

impeller/entity/contents/filters/gaussian_blur_filter_contents.cc

@@ -12,7 +12,9 @@
 #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 {
@@ -87,7 +89,6 @@ 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;
@@ -149,7 +150,6 @@ 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()