Correct default blend mode, fix solid stroke shader to respect premultiplied source colors (#69)

Author: bdero

impeller/entity/contents/content_context.h

@@ -40,7 +40,7 @@ using ClipPipeline = PipelineT<SolidFillVertexShader, SolidFillFragmentShader>;
 
 struct ContentContextOptions {
   SampleCount sample_count = SampleCount::kCount1;
-  Entity::BlendMode blend_mode = Entity::BlendMode::kSource;
+  Entity::BlendMode blend_mode = Entity::BlendMode::kSourceOver;
 
   struct Hash {
     constexpr std::size_t operator()(const ContentContextOptions& o) const {

impeller/entity/shaders/solid_stroke.frag

@@ -8,6 +8,5 @@ in float v_pen_down;
 out vec4 frag_color;
 
 void main() {
-  frag_color = stroke_color;
-  frag_color.a *= floor(v_pen_down);
+  frag_color = stroke_color * floor(v_pen_down);
 }