Incorrect license

Add a test for the Comment constructor.

Fixed a number of smoothstep() UB cases in the shaders

Fixed the red rectangle in servo#427, but the text is still broken (to be addressed later).
The change is similar to servo/webrender@b89ecd1, now all `smoothstep` calls should be correct.

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Text transformation fixes

Fixes servo#427  well, mostly...
Looks like the transformed clips (servo#498) need to be implemented for this to work fully correct.

Also adds a bit of comments to GLSL code to understand what's going on (please check if I misunderstood stuff and documented it wrong!).

The errors spotted:
  1. `layer.screen_vertices` was fetched with wrong indices
  2. `untransform` would use undefined value of `t` if the layer plane is perpendicular to the view
  3. text vertex shaders were missing a division by `vi.local_pos.z` for the layer coordinates
  4. `prim.local_rect` was used for the text_run local rectangle, instead of `local_rect`

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Local rectangle clipping for transformed primitives

Finishes servo#427 implementation, now the text is cut evenly and not aligned to tile borders.
Also refactored `build_bounding_rect()` as well as a bit of `assign_prims_to_screen_tiles()`.

![textrun-comparison3](https://cloud.githubusercontent.com/assets/107301/19860234/5ce0c85a-9f5e-11e6-82f2-d72f19e4641a.png)

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Set the cpp flag on the cc builder in build.rs

Fixes a build error when cross-compiling from linux to android:
```
CC_armv7-linux-androideabi = None
CC_armv7_linux_androideabi = None
HOST_CC = None
CC = None
CFLAGS_armv7-linux-androideabi = None
CFLAGS_armv7_linux_androideabi = None
HOST_CFLAGS = None
CFLAGS = Some("--sysroot /home/servo/android/ndk/r12b/android-ndk-r12b/platforms/android-18/arch-arm -I/home/servo/android/ndk/r12b/android-ndk-r12b/sources/android/support/include")
running: "arm-linux-androideabi-gcc" "-O0" "-ffunction-sections" "-fdata-sections" "-fPIC" "--sysroot" "/home/servo/android/ndk/r12b/android-ndk-r12b/platforms/android-18/arch-arm" "-I/home/servo/android/ndk/r12b/android-ndk-r12b/sources/android/support/include" "-g" "-march=armv7-a" "-march=armv7-a" "-mthumb" "-mfpu=vfpv3-d16" "-mfloat-abi=softfp" "-I" "/home/servo/servo/target/armv7-linux-androideabi/debug/build/mozjs_sys-6ea9cc1d03743689/out/dist/include" "-Wall" "-Wextra" "-fPIC" "-fno-rtti" "-std=c++11" "-include" "/home/servo/servo/target/armv7-linux-androideabi/debug/build/mozjs_sys-6ea9cc1d03743689/out/js/src/js-confdefs.h" "-Wno-c++0x-extensions" "-Wno-return-type-c-linkage" "-Wno-unused-parameter" "-DJS_NO_JSVAL_JSID_STRUCT_TYPES=" "-o" "/home/servo/servo/target/armv7-linux-androideabi/debug/build/mozjs-3864977b9602d849/out/src/jsglue.o" "-c" "src/jsglue.cpp"
cargo:warning=In file included from /home/servo/servo/target/armv7-linux-androideabi/debug/build/mozjs_sys-6ea9cc1d03743689/out/dist/include/mozilla/Attributes.h:12:0,
cargo:warning=                 from /home/servo/servo/target/armv7-linux-androideabi/debug/build/mozjs_sys-6ea9cc1d03743689/out/dist/include/mozilla/Assertions.h:16,
cargo:warning=                 from /home/servo/servo/target/armv7-linux-androideabi/debug/build/mozjs_sys-6ea9cc1d03743689/out/dist/include/mozilla/AlreadyAddRefed.h:12,
cargo:warning=                 from /home/servo/servo/target/armv7-linux-androideabi/debug/build/mozjs_sys-6ea9cc1d03743689/out/dist/include/jsapi.h:12,
cargo:warning=                 from src/jsglue.cpp:15:
cargo:warning=/home/servo/servo/target/armv7-linux-androideabi/debug/build/mozjs_sys-6ea9cc1d03743689/out/dist/include/mozilla/Compiler.h:49:21: fatal error: cstddef: No such file or directory
cargo:warning= #  include <cstddef>
cargo:warning=                     ^
cargo:warning=compilation terminated.
exit code: 1
```
The error shows that the C compiler chain rather than the C++ compiler chain is being used, even though it's compiling a `.cpp` file.

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