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

release/scripts/startup/bl_ui/properties_data_lamp.py

@@ -103,6 +103,13 @@ class DATA_PT_lamp(DataButtonsPanel, Panel):
                 sub.prop(lamp, "linear_attenuation", slider=True, text="Linear")
                 sub.prop(lamp, "quadratic_attenuation", slider=True, text="Quadratic")
 
+            elif lamp.falloff_type == 'INVERSE_COEFFICIENTS':
+                col.label(text="Inverse Coefficients:")
+                sub = col.column(align=True)
+                sub.prop(lamp, "constant_coefficient", text="Constant")
+                sub.prop(lamp, "linear_coefficient", text="Linear")
+                sub.prop(lamp, "quadratic_coefficient", text="Quadratic")
+
             col.prop(lamp, "use_sphere")
 
         if lamp.type == 'AREA':