Full Inverse-Quadratic-Equation Lamp Falloff

This patch adds a new `falloff_type` ('Inverse Coefficients') for Lamps in
Blender-Internal and GLSL.

The current falloff modes use a formula like this inverse-square one:

`I = E � (D^2 / (D^2 + Q � r^2))`

While such a formula is simple for 3D-artists to use, it's algebraically
cumbersome to work with. Game-designers authoring their own shaders
could benefit much more by having direct control of falloff-coefficients:

`I = E � (1.0 / (coefC + coefL � r + coefQ � r^2))`

In this mode, the `distance` parameter is unused (except for 'Sphere'
mode); instead relying on the designer to mathematically-model the
falloff-behavior.

The UI has been patched like so:
{F153843}

Reviewers: brecht, psy-fi

Reviewed By: psy-fi

Subscribers: brita_, antidote, campbellbarton, psy-fi

Differential Revision: https://developer.blender.org/D1194