command_buffer: Implement EXT_blend_func_extended

gpu/command_buffer/service/disk_cache_proto.proto

 syntax = "proto2";
 option optimize_for = LITE_RUNTIME;
 
 message ShaderVariableProto {
   optional uint32 type = 1;
   optional uint32 precision = 2;
   optional string name = 3;
   optional string mapped_name = 4;
   optional uint32 array_size = 5;
   optional bool static_use = 6;
   repeated ShaderVariableProto fields = 7;
   optional string struct_name = 8;
 }
 
 message ShaderAttributeProto {
   optional ShaderVariableProto basic = 1;
   optional int32 location = 2;
 }
 
 message ShaderUniformProto {
   optional ShaderVariableProto basic = 1;
 }
 
 message ShaderVaryingProto {
   optional ShaderVariableProto basic = 1;
   optional int32 interpolation = 2;
   optional bool is_invariant = 3;
 }
 
+message ShaderOutputVariableProto {
+  optional ShaderVariableProto basic = 1;
+  optional int32 location = 2;
+}
+
 message ShaderProto {
   optional bytes sha = 1;
   repeated ShaderAttributeProto attribs = 2;
   repeated ShaderUniformProto uniforms = 3;
   repeated ShaderVaryingProto varyings = 4;
+  repeated ShaderOutputVariableProto output_variables = 5;
 }
 
 message GpuProgramProto {
   optional bytes sha = 1;
   optional uint32 format = 2;
   optional bytes program = 3;
 
   optional ShaderProto vertex_shader = 4;
   optional ShaderProto fragment_shader = 5;
 }