Avoids shadowing, makes it easier to follow which variables are members, which are local.
Warnings from clang:
```
<scratch space>:47:1: note: expanded from here
COLLECTION_PROPERTY_LOOKUP_INT_false
^
source/blender/makesrna/intern/RNA_blender_cpp.h:111:74: note: expanded from macro 'COLLECTION_PROPERTY_LOOKUP_INT_false'
inline static int sname##_##identifier##_lookup_int_wrap(PointerRNA *ptr, int key, PointerRNA *r_ptr) \
^
source/blender/makesrna/intern/RNA_blender_cpp.h:193:16: note: declared here
PointerRNA ptr;
^
source/blender/makesrna/intern/RNA_blender_cpp.h:38795:2: warning: parameter 'ptr' shadows member inherited from type 'Pointer' [-Wshadow-field]
COLLECTION_PROPERTY(MaskLayers, MaskLayer, Mask, layers, false, false, false)
^
source/blender/makesrna/intern/RNA_blender_cpp.h:176:5: note: expanded from macro 'COLLECTION_PROPERTY'
COLLECTION_PROPERTY_LOOKUP_STRING_##has_lookup_string(sname, identifier) \
^
<scratch space>:60:1: note: expanded from here
COLLECTION_PROPERTY_LOOKUP_STRING_false
^
source/blender/makesrna/intern/RNA_blender_cpp.h:139:77: note: expanded from macro 'COLLECTION_PROPERTY_LOOKUP_STRING_false'
inline static int sname##_##identifier##_lookup_string_wrap(PointerRNA *ptr, const char *key, PointerRNA *r_ptr) \
^
source/blender/makesrna/intern/RNA_blender_cpp.h:193:16: note: declared here
PointerRNA ptr;
^
```