od-unix: add Scale2x GLSL scaler to the shader pipeline

od-unix: add Scale2x GLSL scaler to the shader pipeline

The Filter page's filter selector only offered "None" (GitHub issue
#4): the generic gfx_filter config option cannot carry named shaders
outside the Windows Direct3D path, and the Unix GL presenter had no
scaler stage.

Carry the selection as a new unix.gfx_shader target option (parsed and
saved in od-unix/config.cpp, no generic code changes) and implement
Scale2x/EPX as a stage in the GL presenter's fragment shader, selected
through a uniform. The scaler forces nearest-neighbour sampling, runs
before the color/scanline/noise stages, and applies to the emulation
frame only, leaving the status line untouched. The launcher's Filter
selector offers Scale2x when the shader pipeline is available, and the
value round-trips through the config (covered by the roundtrip smoke).