Add impellerc options to compile OpenGL Desktop and ES variant shader…

…s. (flutter#139)

Also parameterize all unit-tests so all backends are automatically tested.

impeller/compiler/BUILD.gn

@@ -9,12 +9,20 @@ impeller_component("compiler_lib") {
     "code_gen_template.h",
     "compiler.cc",
     "compiler.h",
+    "compiler_backend.cc",
+    "compiler_backend.h",
     "include_dir.h",
+    "includer.cc",
+    "includer.h",
     "logger.h",
     "reflector.cc",
     "reflector.h",
+    "source_options.cc",
+    "source_options.h",
     "switches.cc",
     "switches.h",
+    "types.cc",
+    "types.h",
     "utilities.cc",
     "utilities.h",
   ]
@@ -52,7 +60,11 @@ impeller_component("compiler_unittests") {
 
   output_name = "impellerc_unittests"
 
-  sources = [ "compiler_unittests.cc" ]
+  sources = [
+    "compiler_test.cc",
+    "compiler_test.h",
+    "compiler_unittests.cc",
+  ]
 
   deps = [
     ":compiler_lib",

impeller/compiler/code_gen_template.h

@@ -53,7 +53,7 @@ struct {{camel_case(shader_name)}}{{camel_case(shader_stage)}}Shader {
 
   static constexpr auto kResource{{camel_case(buffer.name)}} = ShaderUniformSlot<{{buffer.name}}> { // {{buffer.name}}
     "{{buffer.name}}",     // name
-    {{buffer.msl_res_0}}u, // binding
+    {{buffer.ext_res_0}}u, // binding
   };
 {% endfor %}
 {% endif %}
@@ -91,8 +91,8 @@ struct {{camel_case(shader_name)}}{{camel_case(shader_stage)}}Shader {
 
   static constexpr auto kResource{{camel_case(sampled_image.name)}} = SampledImageSlot { // {{sampled_image.name}}
     "{{sampled_image.name}}",      // name
-    {{sampled_image.msl_res_0}}u,  // texture
-    {{sampled_image.msl_res_1}}u,  // sampler
+    {{sampled_image.ext_res_0}}u,  // texture
+    {{sampled_image.ext_res_1}}u,  // sampler
   };
 {% endfor %}
 {% endif %}