glsl: allow ForceGLSLVersion to override #version directives

glsl: allow ForceGLSLVersion to override #version directives

Previously, the ctx->Const.ForceGLSLVersion setting only worked if
the shader lacked a #version directive.  Now, the ForceGLSLVersion
setting will override the #version directive too.

This change should be safe since it should be rare to have an app
that has a mix of shader versions and we only wanted to override
the #version for shaders which lacked the #version directive.

Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>