BLI: add conversion from float2 to float3

Previously float2 was converted to float3 by implicitly converting to a
float pointer first, which was then passed to the float3 constructor.
This leads to uninitialized memory in the z component of the new float3.

source/blender/blenlib/BLI_float2.hh

@@ -47,6 +47,11 @@ struct float2 {
     return &x;
   }
 
+  operator float3() const
+  {
+    return float3(x, y, 0.0f);
+  }
+
   float length() const
   {
     return len_v2(*this);