canvas-dnd: Define function type the usual way

Defining an function type inline as a function parameter is a awkward syntax and not used in our codebase except for this one occurence.

Although valid C, it confuses our style check tools, breaking the CI style check [1] and failing to align parameters.

Instead, let's define a named function type to use as parameter type, and fix parameter misalignment.

[1] 831203e9