[Impeller] Disable blending for Source blend mode (#43162)

All of the backends already respect this property for the first color
attachment in the pipeline. ~~Our Vulkan backend disables blending for a
whole pipeline if the first color attachment has blending turned off.~~
This has been under our nose but I haven't checked it until now.

HSR might not be working on some Vulkan & OpenGLES drivers because of
this.

Author: bdero

impeller/entity/contents/content_context.cc

@@ -42,6 +42,7 @@ void ContentContextOptions::ApplyToPipelineDescriptor(
       color0.src_color_blend_factor = BlendFactor::kZero;
       break;
     case BlendMode::kSource:
+      color0.blending_enabled = false;
       color0.dst_alpha_blend_factor = BlendFactor::kZero;
       color0.dst_color_blend_factor = BlendFactor::kZero;
       color0.src_alpha_blend_factor = BlendFactor::kOne;