Set a minimum of 0.01 for proximity fade to avoid undefined behavior in shader

[clayjohn]

Caught while running Calinou's test suite. Currently we have some undefined behaviour when using proximity fade with a distance of 0.

godot/scene/resources/material.cpp

Line 1372 in 6128206

code += " ALPHA*=clamp(1.0-smoothstep(world_pos.z+proximity_fade_distance,world_pos.z,VERTEX.z),0.0,1.0);\n";

https://registry.khronos.org/OpenGL-Refpages/gl4/html/smoothstep.xhtml

Results are undefined if edge0 ≥ edge1.

[akien-mga]

Thanks!