Add support for `coord_polar()` in `geom_raster()`

Code snippet:

```python
import numpy as np
from scipy.stats import multivariate_normal
from lets_plot import *
LetsPlot.setup_html()
np.random.seed(42)
n = 25
x = np.linspace(-1, 1, n)
y = np.linspace(-1, 1, n)
X, Y = np.meshgrid(x, y)
mean = np.zeros(2)
cov = [[1, -.5], [-.5, 1]]
rv = multivariate_normal(mean, cov)
Z = rv.pdf(np.dstack((X, Y)))

data = {
'x': X.flatten(),
'y': Y.flatten(),
'z': Z.flatten()
}

p = ggplot(data) \
+ geom_raster(aes(x='x', y='y', fill='z')) \
+ scale_fill_gradient(low='#54278f', high='#f2f0f7')

gggrid([
p ,
p + coord_polar(),
])
```

Output: