Stencil-canvas-related adjustments

[SleepyG11]

This PR adjust 2 things related to stencils and canvases.

1. Fixed a crash which happens when love.graphics.getCanvas returns nil.
2. Adds canvas argument to SMODS.reload_stencil_stack as shortcut for setting canvas together with depthstencil.

-- old way: 2 setCanvas calls
love.graphics.setCanvas(new_canvas)
SMODS.reload_stencil_stack()

-- new way: 1 setCanvas call
-- first argument is full stencil reload, second argument is canvas we want to set
SMODS.reload_stencil_stack(nil, new_canvas)

Additional Info:

• I didn't modify api's or I've made a PR to the wiki repo.
• I didn't modify api's or I've updated lsp definitions.
• I didn't make new lovely files or all new lovely files have appropriate priority.