Fix #131139: Small brush radius and alpha texture degrades performance

The call to `sculpt_apply_texture` is not thread safe and as such is a
major bottleneck in brush stroke processing. In previous versions, we
avoided calculating this if the vertex position was outside of the brush
radius.

To fix this, we add a check for the factor already being 0 and prevent
further texture calculations.

Pull Request: https://projects.blender.org/blender/blender/pulls/131256

source/blender/editors/sculpt_paint/sculpt.cc

@@ -6939,6 +6939,9 @@ void calc_brush_texture_factors(const SculptSession &ss,
   }
 
   for (const int i : verts.index_range()) {
+    if (factors[i] == 0.0f) {
+      continue;
+    }
     float texture_value;
     float4 texture_rgba;
     /* NOTE: This is not a thread-safe call. */
@@ -6963,6 +6966,9 @@ void calc_brush_texture_factors(const SculptSession &ss,
   }
 
   for (const int i : positions.index_range()) {
+    if (factors[i] == 0.0f) {
+      continue;
+    }
     float texture_value;
     float4 texture_rgba;
     /* NOTE: This is not a thread-safe call. */