testVisibility(): check for intersections with nodes, not with node->geoms.

The shadow below the "soccer ball" is now visible
Fix a bug with the Model-view transform.

IntersectionData::dist wasn't handled correctly when transforming between
world-space and object space. The scaling required there was
just the opposite :)
Implement bilinear filtering, and use it in BitmapTexture and CubemapEnvironment.
Move saveEXR() from BitmapExt (in hdr2exr) into the base Bitmap class.

trinity can save EXRs now, too.
Add a capability to do screenshots of the render results. F12 to save in .BMP format, Shift+F12 - in .EXR.
Conformize GPL headers.
Make the rendering region bucket markings more visible.

The "cursor", which iterates the render regions was previously only
blue, 1px thick. This made it nearly invisible against some objects.
Now the cursor gets 1px light-grey outline on each side, so it stands
out regardless of the actual image contents.
Speed up rendering (don't anti-alias overexposed areas).

The old code in the tooDifferent() function looked for color differences
of more than 0.1 in any of the RGB channels - in which case the colors
were deemed "too different" unconditionally, and an AA treatment to
the respective pixel was employed.

This doesn't make sense for the overexposed areas, which happen a lot with
the current scene, because of the HDR background and the reflective/glossy
surfaces. There, the relative difference between neighbouring overexposed
pixels is similar to non-overexposed pixels which don't require AA.
However, since their magnitude is larger, the absolute color difference
is proportionally larger as well, and they almost always qualify for
the AA pass. This results in needless oversampling in the overexposed
areas, which is moot anyway; they are overexposed; there's no detail there.

The new code doesn't consider it interesting when the darker of the two
pixels is more than 1.33 in absolute brightness (technically, 1.0 should
be correct here as well, but let's be a little on the safe side).