If an object named for example `Suzanne` is converted to Gpencil, a material called `Suzanne_Fill` will be created for the gpencil fill. When this material already exists, the new material will be called `Suzanne_Fill.001` and the operator will not see that this material is already present the next iteration. This leads to a new material being created for every polygon. This commit changes the code to search for a material starting with `ObjectName_Fill` instead of being equal to. Reviewed By: filedescriptor, antoniov Differential Revision: https://developer.blender.org/D11067