Override shader path by default for examples and wrench

[kvark]

Examples and wrench main use case is assisting developers, which built WR locally in the first place. I suggest us having these binary to use local shaders by default, so that we can iterate quickly without putting the command line every time. Perhaps, even have a compile feature to include shaders in the build that is only used by Servo/Gecko?
We could write the shader path by the build.rs somewhere, then read in the examples/wrench.