Make TF Feedback buffer mode a GL-level variable.

Don't query this as an Impl method, since it exists on the GL level.
Also some related refactorings and cleanups.

BUG=angleproject:1123

Change-Id: I3610bc0db2bcaa96408506e06a65a2f4dab93150
Reviewed-on: https://chromium-review.googlesource.com/293761
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>
diff --git a/src/libANGLE/renderer/d3d/DynamicHLSL.cpp b/src/libANGLE/renderer/d3d/DynamicHLSL.cpp
index 0cfa42a..142ef6d 100644
--- a/src/libANGLE/renderer/d3d/DynamicHLSL.cpp
+++ b/src/libANGLE/renderer/d3d/DynamicHLSL.cpp
@@ -736,10 +736,14 @@
     }
 }
 
-bool DynamicHLSL::generateShaderLinkHLSL(const gl::Data &data, InfoLog &infoLog, int registers,
+bool DynamicHLSL::generateShaderLinkHLSL(const gl::Data &data,
+                                         InfoLog &infoLog,
+                                         int registers,
                                          const VaryingPacking packing,
-                                         std::string &pixelHLSL, std::string &vertexHLSL,
-                                         ShaderD3D *fragmentShader, ShaderD3D *vertexShader,
+                                         std::string &pixelHLSL,
+                                         std::string &vertexHLSL,
+                                         const ShaderD3D *fragmentShader,
+                                         const ShaderD3D *vertexShader,
                                          const std::vector<std::string> &transformFeedbackVaryings,
                                          std::vector<LinkedVarying> *linkedVaryings,
                                          std::map<int, VariableLocation> *programOutputVars,
@@ -1086,7 +1090,8 @@
     return true;
 }
 
-void DynamicHLSL::defineOutputVariables(ShaderD3D *fragmentShader, std::map<int, VariableLocation> *programOutputVars) const
+void DynamicHLSL::defineOutputVariables(const ShaderD3D *fragmentShader,
+                                        std::map<int, VariableLocation> *programOutputVars) const
 {
     const std::vector<sh::Attribute> &shaderOutputVars = fragmentShader->getActiveOutputVariables();