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
f1cf706758