Use dedicated params dict for each canvas type in WPT test generator

Some generated canvas tests have different implementations for
different canvas types (HTML element, OffscreenCanvas or
OffscreenCanvas in a worker). To support this, the code generator
rendered the test's code three times, into different template
parameters: code_element, code_offscreen and code_worker. Likewise,
grid tests can have different variants for different canvas types,
so the test generator used three different params to store the variant
list: element_variants, offscreen_variants and worker_variants.

This strategy wasn't scalable though. In theory the test implementation
is free to customize any parameter based on the canvas type and thus,
we'd need to carry three different versions of each of these param.
We can actually do this automatically, for all parameters, by simply
cloning the whole params list, once for each canvas type. This CL
does this by using a dict `_canvas_type_params`, which stores the
params specific to each canvas type.

Bug: 40207206
Change-Id: I733dfb50865eb6f0ca263bc1805ffdba37cdb54d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6204421
Reviewed-by: Andres Ricardo Perez <andresrperez@chromium.org>
Commit-Queue: Jean-Philippe Gravel <jpgravel@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1415019}