texture2DRect support in gles2_cmd_copy_texture_chromium

Some support for GL_TEXTURE_RECTANGLE_ARB was implemented,
but many cases, including using proper GLSL sampling functions,
were missing. This fixes some WebGL conformance tests on MacOS.

Bug: chromium:757974
Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: I645dc545643c4cd0f392d8000550c098c7b109f6
Reviewed-on: https://chromium-review.googlesource.com/944185
Commit-Queue: Alexis Hétu <sugoi@chromium.org>
Reviewed-by: Antoine Labour <piman@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#540836}

gpu/command_buffer/service/gles2_cmd_copy_texture_chromium.cc

@@ -398,6 +398,9 @@ std::string GetFragmentShaderSource(const gl::GLVersionInfo& gl_version_info,
       case GL_TEXTURE_EXTERNAL_OES:
         source += "#define TextureLookup texture2D\n";
         break;
+      case GL_TEXTURE_RECTANGLE_ARB:
+        source += "#define TextureLookup texture2DRect\n";
+        break;
       default:
         NOTREACHED();
         break;