Fix shaders failing to compile with Metal renderer

[prime-optimal]

Fixes #85.

Root cause

The Metal Shading Language has no scalar overload of distance() — only float2/3/4 (and half variants). OBS's Metal backend (libobs-metal) transpiles effect-HLSL intrinsics verbatim, so PSGradient and PSRange fail to compile with:

program_source:19:30: error: call to 'distance' is ambiguous
    float lerp_t = saturate((distance(vert_in.tex.y, uniforms.grad_center) - uniforms.grad_offset) / uniforms.grad_height);

One failed shader makes gs_effect_create_from_file return NULL for the whole effect, so every technique (including Solid) stops rendering — matching the "no visual in any mode" symptom. The OBS log only shows the generic MTLDevice failed to create shader library and function; I recovered the real MTLCompiler error by instrumenting MetalShader.swift in a local libobs-metal build.

Fix

Replace scalar distance(a, b) with abs(a - b) in PSGradient and PSRange. Semantically identical for scalars and valid in HLSL, GLSL, and MSL, so D3D11/OpenGL behavior is unchanged.

Verification

macOS 26.4, Apple Silicon, OBS 32.1.2 with Renderer=Metal:

• Before: 4× device_vertexshader_create: Device error compiling shader, Pass (0) <> missing pixel shader!, per-frame effect_setval_inline: invalid param spam, source screenshot has 0 painted pixels.
• After: zero shader errors in the log; waveform renders correctly in mono and stereo (checked Gradient/Range paths; stereo draws both channels independently).

Everything else in gradient.effect transpiles cleanly on 32.1.2 (uniform default initializers, bool uniform, mul(), scalar saturate/lerp, all six techniques, GS_TRISTRIP/LINESTRIP/TRIS draws).

[phandasm]

Thanks!