spearmint: Use flare shader specific by BSP surface

Quake 3 maps always has it set to 'flareShader'. SoF2/JK2/JA maps specify
different shaders.

Raven put the flare shaders on brushes.  q3map2 allows specifying on light
entities "_flareShader <shadername>" and in shaders "q3map_flare[Shader] <shadername>".