Swizzle and implicit binding predicate fixes

[rtabbara]

dr.cross : Cast return type back to original type

• Use of swizzle operator will return a dr.jit array type. This means for example swizzling a mi.Vector3f returns a dr.*.Array3f

For implicit conversion predicate from dr.jit types, also check dimensions are compatible

• Previously types such as Array4f -> Matrix4<Color3f> would return true as we're just simply checking that the first dimension aligns

[rtabbara]

Also addresses Mitsuba issue 1280

[wjakob]

Wouldn't it be preferable to fix this by changing swizzling so that it preserves the type when the number of array dimensions remains the same?

[Speierers]

Somehow this patch prevents casting a Matrix<4, Float> into a Matrix<4, Color<3, Float>> as in the following example:

import mitsuba as mi
mi.set_variant('llvm_ad_rgb_polarized')

print(mi.Spectrum(mi.mueller.depolarizer(1)))

[rtabbara]

@wjakob I've updated the PR so that instead the swizzle operation returns the original type if the swizzle size is compatible

@Speierers I think I've fixed this now and added some corresponding tests related to this

[njroussel]

There will always be a tradeoff between an "excessive" check in the implicit cast predicate, and letting it fail during the actual construction attempt. These change are fair for now, in my opinion.