Check before calling _rescale_discrete_levels

[ianthomas23]

Fixes #1084.

In _interpolate_alpha I wasn't checking before calling _rescale_discrete_levels the same as is done in _interpolate, i.e.

datashader/datashader/transfer_functions/__init__.py

Lines 278 to 279 in 59d1d2d

	if rescale_discrete_levels and discrete_levels is not None: # Only valid for how='eq_hist'
	span = _rescale_discrete_levels(discrete_levels, span)

Test added, and I have checked that there are no other places that _rescale_discrete_levels is called that could cause a similar problem.

[codecov]

Codecov Report

Merging #1085 (b5d5dfc) into master (59d1d2d) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1085      +/-   ##
==========================================
+ Coverage   83.35%   83.36%   +0.01%     
==========================================
  Files          34       34              
  Lines        7491     7491              
==========================================
+ Hits         6244     6245       +1     
+ Misses       1247     1246       -1     

Impacted Files	Coverage Δ
datashader/transfer_functions/__init__.py	86.61% <100.00%> (+0.19%)	⬆️

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