v0.7 test 1
* improved disc sampling
* added in polarisation simulation
* specular tag of phong removed; use ior instead.
* added bloom tag to camera tag.
* added in precedence for transparent specular materials
* some small speedups for kdtree traversal
* added in blend material
* added diffuse_transmitter material
* added autofocus option for camera
* (re)added ply model loading
* fucked around with mesh loading code
* meshes should use less memory now
* raises fatal error on unexpected element when parsing scene xml
* throw error on texture missing, instead of issuing warning
* changed how burn is used, the burn value is now based on the average image luminance instead of the max over all the pixels.
* Added in new 'medium' object and changed how specular materials work.
* changed absorption to absorption_coefficient_spectrum
* added glossy_transparent material
* added uniform spectrum type
* Added internal mesh instancing
* raises error on non-orthogonal rotation matrix
* added SSS and participating medía in general.
* added normal_smoothing boolean element to mesh element.
* removed enable_normal_smoothing element from model element
* added fatal error if no lights in scene
* possibly fixed specular bump mapping bug (was not binding uv coords)
* removed gain from texture elements, using a, b, c polynomial coefficients instead
* fixed bug where reverse gamma correction was applied múltiple times to same texture.