Fix #103398: Fix Icon sampler initialization in Metal backend.

Resolves issue with nearest filtering on UI Icons. Note that as
Metal does not support LOD bias as a parameter on a sampler
object, the original code has been modified to perform LOD
biasing at the shader level.

As GPU_SAMPLER_ICON is not  widely used, it is more
efficient to apply directly to the  affected shaders, rather
than workaround passing in the sampler LOD bias as a
separate value e.g. uniform or push constant.

Original PR feedback addressed to also refactor ICON
shaders to use consistent style for single and multi
Icon rendering.

Authored by Apple: Michael Parkin-White

Ref #96261
Pull Request #105145

source/blender/gpu/shaders/gpu_shader_icon_frag.glsl

@@ -8,6 +8,10 @@
 
 void main()
 {
+  /* Sample texture with LOD BIAS. Used instead of custom lod bias in GPU_SAMPLER_ICON. */
+  fragColor = texture(image, texCoord_interp, -0.5) * finalColor;
+
+#ifdef DO_CORNER_MASKING
   /* Top-left rounded corner parameters. */
   const float circle_radius_outer = 0.1;
   const float circle_radius_inner = 0.075;
@@ -18,7 +22,6 @@ void main()
   const float mask_transparency = 0.25;
 
   vec2 circle_center = vec2(circle_radius_outer - text_width, 0.5);
-  fragColor = texture(image, texCoord_interp) * color;
 
   /* radius in icon space (1 is the icon width).  */
   float radius = length(mask_coord_interp - circle_center);
@@ -39,4 +42,5 @@ void main()
   }
 
   fragColor = mix(vec4(0.0), fragColor, max(mask_transparency, mask));
+#endif
 }