vbo: fix glVertexAttrib(index=0)

vbo: fix glVertexAttrib(index=0)

Depending on which extension or GL spec you read the behavior of
glVertexAttrib(index=0) either sets the current value for generic
attribute 0, or it emits a vertex just like glVertex().  I believe
it should do either, depending on context (see below).

The piglit gl-2.0-vertex-const-attr test declares two vertex attributes:
  attribute vec2 vertex;
  attribute vec4 attr;
and the GLSL linker assigns "vertex" to location 0 and "attr" to location 1.
The test passes.

But if the declarations were reversed such that "attr" was location 0 and
"vertex" was location 1, the test would fail to draw properly.

The problem is the call to glVertexAttrib(index=0) to set attr's value
was interpreted as glVertex() and did not set generic attribute[0]'s value.
Interesting, calling glVertex() outside glBegin/End (which is effectively
what the piglit test does) does not generate a GL error.

I believe the behavior of glVertexAttrib(index=0) should depend on
whether it's called inside or outside of glBegin/glEnd().  If inside
glBegin/End(), it should act like glVertex().  Else, it should behave
like glVertexAttrib(index > 0).  This seems to be what NVIDIA does.

This patch makes two changes:

1. Check if we're inside glBegin/End for glVertexAttrib()
2. Fix the vertex array binding for recalculate_input_bindings().  As it was,
   we were using &vbo->currval[VBO_ATTRIB_POS], but that's interpreted
   as a zero-stride attribute and doesn't make sense for array drawing.

No Piglit regressions.  Fixes updated gl-2.0-vertex-const-attr test and
passes new gl-2.0-vertex-attrib-0 test.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=101941
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Charmaine Lee <charmainel@vmware.com>