This avoid some annoying focus stealing in some situations, and looks
nicer in general.

If the first type of breakpoint to be hit wasn't "Vertex Loaded", the
input_vertex would contain garbage, which would be passed to the shader
interpreter and ocasionally cause crashes.

Renamed "Iteration index" to the (hopefully) more intuitive "Cycle Index".
Added flexible space at the bottom of the widget.

Introduce a shader tracer to allow inspection of input/output values for each processed instruction.

Now supports dumping the current shader and recognizes a larger number of output semantics.

- Also renames "vertex_shader.*" to "shader_interpreter.*"

Includes more opcodes to implement in the future.