Bidirectional Path Tracing support for pbrt Release 1.02

                          By MLJack 
                          mljack at gmail dot com
                          mljack.googlepages.com

It's a migration of RenderPark's implementation.

Reference:

http://graphics.stanford.edu/papers/veach_thesis/
http://www.cs.kuleuven.ac.be/~graphics/RENDERPARK/

Feathers/Limitations:

* Bidirectional Path Tracing with Russian Roulette
* Particle Tracing
* Materials: matte/phong/glass/mirror
* Only support area light. (Also see the interface changes)
* Only support perspective camera. (Also see the interface changes)
* BidirectionalPathTracing plugin need image4BidirectionalPT plugin.
* ParticleTracing plugin need image4PaticleTracing plugin.
* Pinhole camera (no DOF)
* Vacuum only (no participating media)
* Use the default cropwindow parameter in any image plugins.
* Use the following filter configuration:
        PixelFilter "box" "float xwidth" [0.5] "float ywidth" [0.5]

Samples:
* See files in the _scene directory.

License:
* All Rights Reserved.
* For educational use only; commercial use expressly forbidden.
* NO WARRANTY, express or implied, for this software.
* (See file License.txt for complete license)

If you have any problems, notice errors, or make any patches, please let me know.

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The BidirectionalPT plugin depends on the following changes/interfaces.
It call AddSample(...) directly to store samples in the screen.

Some major changes:

New global variables:

int PBRT_DEBUG:                 Debugging state of system
Scene* scene:                   Plugins can communicate with each others
                                not only through interface of core, but
                                also interfaces of other plugins.
int PBRT_FILE_CurrentImage:     Store the current output image index.

New classes:

Spectrum_double:                Double-precision floating point number to prevent underflow.


New interfaces:

In class SurfaceIntegrator:
IsLastSampleLiValid(...):       Return last return by Li(...) should be stored in the image.

In class Camera:
GenerateSample(...):            Convert a sample from world space to camera space.
getEyePoint(...):               Return eye position in world space.
getEyeNormal(..):               Return eye direction in world space.
isVisibleFromEye(...):          Return the visibility between eye and a world space position.

In class Light:
Sample_Point(...):              Return a point sample in the light surface.
Sample_Direction(...):          Return a direction sample in a point of light surface.
LemitEval(...):                 Return the emitting radiance.
PointPdfEval(...):              Return the PDF of point sampling.
DirectionPdfEval(...):          Return the PDF of direction sampling.


New parameters:

In Intersection::GetBSDF(...)
Spectrum *ScatterPower:         Get the total scattering power.
                                It can be used in Russian Roulette.

New class members:

In class Film:
float R, xWidth, yHeight, xPixelWidth, yPixelHeight, Apixel:  Physical parameters of the screen

In class Perspective:
Transform WorldToRaster:       Transform from world space to pixel space