geom_raster() fallback for non-Cartesian coords

[teunbrand]

This PR aims to fix #5503.

Briefly, geom_raster() now falls back to drawing as geom_rect() when coordinates are non-linear.
A small outline in the same colour as the fill aesthetic is added to prevent anti-aliasing.

devtools::load_all("~/packages/ggplot2/")
#> ℹ Loading ggplot2

ggplot(faithfuld, aes(eruptions, waiting, fill = density)) +
  geom_raster() +
  coord_polar()
#> `geom_raster()` only works with `coord_cartesian()`.
#> ℹ Falling back to drawing as `geom_rect()`.

^{Created on 2024-01-04 with reprex v2.0.2}

[thomasp85]

LGTM