Reverts "[Impeller] disable blending in gaussian intermediate steps. (#51118)" (#51244)

Reverts: #51118
Initiated by: jonahwilliams
Reason for reverting: unexpected performance change

https://flutter-flutter-perf.skia.org/e/?queries=sub_result%3D90th_percentile_frame_rasterizer_time_millis%26sub_result%3D99th_percentile_frame_rasterizer_time_millis%26sub_result%3Daverage_frame_rasterizer_time_millis%26sub_result%3Dworst_frame_rasterizer_time_millis%26test%3Dbackdrop_filter_perf_ios__timeline_summary&selected=commit%3D39677%26name%3D%252Carch%253
Original PR Author: jonahwilliams

Reviewed By: {gaaclarke}

This change reverts the following previous change:
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: 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()