Respect gamma setting of max bitmap textures

[rbrune]

When using linear textures in an Bitmap as an input to e.g. a diffuse color the gamma override setting in the 3ds Max Bitmap dialog is ignored and an sRGB to linear conversion node is inserted.

The corresponding check is done in utilities.cpp / is_linear_texture(BitmapTex* bitmap_tex) function which just checks the file ending against .exr and .hdr. As a first improvement we should check if the input gamma is overridden and set to 1.0.