Add necessary extension for compiling #version 150 fixedcol.frag

* Previously we were implicitly satisfying this by making it a separable
  program.

renderdoc/data/glsl/fixedcol.frag

@@ -29,6 +29,8 @@
 
 #else
 
+//#extension_nongles GL_ARB_explicit_attrib_location : require
+
 // otherwise we use a proper output
 layout (location = 0) out vec4 color_out;