Fix multiply_fragment_source to compile with Apple GLSL compiler.
https://bugzilla.gnome.org/show_bug.cgi?id=616748
"void main () {"
" vec4 basecolor = texture2DRect (base, gl_TexCoord[0].st);"
" vec4 blendcolor = texture2DRect (blend, gl_TexCoord[0].st);"
- " gl_FragColor = (1 - alpha) * basecolor + alpha * basecolor * blendcolor;"
+ " gl_FragColor = (1.0 - alpha) * basecolor + alpha * basecolor * blendcolor;"
"}";
/* lut operations, map luma to tex1d, see orange book (chapter 19) */