mesa: fix GLSL issue preventing use of all 16 generic vertex attributes

Only 15 actually worked before since we always reserved generic[0] as an
alias for vertex position.

The case of vertex attribute 0 is tricky.  The spec says that there is no
aliasing between generic vertex attributes 0..MAX_VERTEX_ATTRIBS-1 and the
conventional attributes.  But it also says that calls to glVertexAttrib(0, v)
are equivalent to glVertex(v).  The distinction seems to be in glVertex-mode
versus vertex array mode.

So update the VBO code so that if the shader uses generic[0] but not gl_Vertex,
route the attribute data set with glVertex() to go to shader input generic[0].

No change needed for the glDrawArrays/Elements() path.

This is a potentially risky change so regressions are possible.  All the usual
tests seem OK though.

1 files changed, 13 insertions, 0 deletions

diff --git a/src/mesa/vbo/vbo_exec_draw.c b/src/mesa/vbo/vbo_exec_draw.c
index 5b2e3550ca8..7204aec1dcb 100644
--- a/src/mesa/vbo/vbo_exec_draw.c
+++ b/src/mesa/vbo/vbo_exec_draw.c
@@ -175,7 +175,20 @@ static void vbo_exec_bind_arrays( GLcontext *ctx )
          exec->vtx.inputs[attr + 16] = &vbo->generic_currval[attr];
       }
       map = vbo->map_vp_arb;
+
+      /* check if VERT_ATTRIB_POS is not read but VERT_BIT_GENERIC0 is read.
+       * In that case we effectively need to route the data from
+       * glVertexAttrib(0, val) calls to feed into the GENERIC0 input.
+       */
+      if ((ctx->VertexProgram._Current->Base.InputsRead & VERT_BIT_POS) == 0 &&
+          (ctx->VertexProgram._Current->Base.InputsRead & VERT_BIT_GENERIC0)) {
+         exec->vtx.inputs[16] = exec->vtx.inputs[0];
+         exec->vtx.attrsz[16] = exec->vtx.attrsz[0];
+         exec->vtx.attrsz[0] = 0;
+      }
       break;
+   default:
+      assert(0);
    }
 
    /* Make all active attributes (including edgeflag) available as