Fix: Large stack allocation in compositor

When COM_EXPORT_GRAPHVIZ is enabled, DebugInfo::graphviz
uses a char[1000000] as local variable. When this function
is called this is allocated on the stack, which has a size
of just 1MB on mac and may cause a stack overflow.

This patch allocates the memory on the heap and frees
the memory at the end of the function.

Reviewed By: LazyDodo

Differential Revision: https://developer.blender.org/D13628

Fix T94142, T94182: Cycles metal broken after pointcloud changes

Missing ccl_private form an older patch.

Differential Revision: https://developer.blender.org/D13612

Initialize the fourth and final instance variable of MemoryProxy

The constructor of MemoryProxy initializes 3 of 4 instances variables.
If a MemoryProxy is constructed and MemoryProxy::free is called
on this instance, buffer_ is undefined and 'delete buffer_;' causes errors.

Although this misuse pattern does not exist in the current codebase
it already tripped up the Address Sanitizer on various occasions
while debugging unrelated problems.

Reviewed By: jbakker

Differential Revision: https://developer.blender.org/D13569