avoid an implicit type conversion in a shader

renderprogs/interactionSM.pixel does not compile with the Mesa
shader compiler due to a an implicit type conversion.

This gives a console warning on a release build and fatally
asserts on an debug build:

0:135(19): error: could not implicitly convert operands to arithmetic operator

Avoid this by changing an int declaration to a float.
Merge pull request #143 from jonathangray/mesa-shader-fix

make a shader work with Mesa by avoiding an implicit type conversion