glsl/linker: Subdivide the first phase of varying assignment.

This patch further subdivides the loop that assigns varying locations
into two phases: one phase to match up the varyings between shader
stages, and one phase to assign them varying locations.

In between the two phases the matched varyings are stored in a new
data structure called varying_matches.  This will free us to be able
to assign varying locations in any order, which will pave the way for
packing varyings.

Note that the new varying_matches::assign_locations() function returns
the number of varying slots that were used; this return value will be
used in a future patch.
glsl/linker: Sort varyings by packing class, then vector size.

This patch paves the way for varying packing by adding a sorting step
before varying assignment, which sorts the varyings into an order that
increases the likelihood of being able to find an efficient packing.

First, varyings are sorted into "packing classes" by considering
attributes that can't be mixed during varying packing--at the moment
this includes base type (float/int/uint/bool) and interpolation mode
(smooth/noperspective/flat/centroid), though later we will hopefully
be able to relax some of these restrictions.  The number of packing
classes places an upper limit on the amount of space that must be
wasted by varying packing, since in theory a shader might nave 4n+1
components worth of varyings in each of m categories, resulting in 3m
components worth of wasted space.

Then, within each packing class, varyings are sorted by vector size,
with vec4's coming first, then vec2's, then scalars, and then finally
vec3's.  The motivation for this order is that it allows all
power-of-two sized varyings to be located at multiples of their
natural alignment (vec4's located on vec4 boundaries, vec2's located
on vec2 boundaries) without wasting space.  Once the vec4's, vec2's,
and scalars have been placed, the vec3's may be either be aligned to
vec4 boundaries (resulting in a wost-case 33% waste of space) or
aligned to scalar boundaries (resulting in some performance loss).

Note that the varyings aren't actually packed yet, merely placed in an
order that will facilitate packing to be introduced by a later patch.
Add lower_packed_varyings
glsl/linker: Pack within compound varyings.

This patch implements varying packing within varyings that are
composed of multiple vectors of size less than 4 (e.g. arrays of
vec2's, or matrices with height less than 4).

Previously, such varyings used up a full 4-wide varying slot for each
constituent vector, meaning that some of the components of each
varying slot went unused.  For example, a mat4x3 would be stored as
follows:

 <----slot1----> <----slot2----> <----slot3----> <----slot4---->  slots
  *   *   *   *   *   *   *   *   *   *   *   *   *   *   *   *
 <-column1->  x  <-column2->  x  <-column3->  x  <-column4->  x   matrix

(Each * represents a varying component, and the "x"s represent wasted
space).  In addition to wasting precious varying components, this
layout complicated transform feedback, since the constituents of the
varying are expected to be output to the transform feedback buffer
contiguously (e.g. without gaps between the columns, in the case of a
matrix).

This change packs the constituents of each varying together so that
all wasted space is at the end.  For the mat4x3 example, this looks
like so:

 <----slot1----> <----slot2----> <----slot3----> <----slot4---->  slots
  *   *   *   *   *   *   *   *   *   *   *   *   *   *   *   *
 <-column1-> <-column2-> <-column3-> <-column4->  x   x   x   x   matrix

Note that matrix columns 2 and 3 now cross a boundary between varying
slots (a characteristic I call "double parking" of a varying).

Since compiler back-ends don't (yet) support this packed layout, the
lower_packed_varyings function is used to rewrite the shader into a
form where each varying occupies a full varying slot.  Later, if we
add native back-end support for varying packing, we can make this
lowering pass optional.
glsl/linker: Pack between varyings.

This patch implements varying packing between varyings.

Previously, each varying occupied components 0 through N-1 of its
assigned varying slot, so there was no way to pack two varyings into
the same slot.  For example, if the varyings were a float, a vec2, a
vec3, and another vec2, they would be stored as follows:

 <----slot1----> <----slot2----> <----slot3----> <----slot4---->  slots
  *   *   *   *   *   *   *   *   *   *   *   *   *   *   *   *
 flt  x   x   x  <vec2->  x   x  <--vec3--->  x  <vec3->  x   x   varyings

(Each * represents a varying component, and the "x"s represent wasted
space).

This change packs the varyings together to eliminate wasted space
between varyings, like so:

 <----slot1----> <----slot2----> <----slot3----> <----slot4---->  slots
  *   *   *   *   *   *   *   *   *   *   *   *   *   *   *   *
 <vec2-> <vec2-> flt <--vec3--->  x   x   x   x   x   x   x   x   varyings

Note that we take advantage of the sort order introduced in previous
patches (vec4's first, then vec2's, then scalars, then vec3's) to
minimize how often a varying is "double parked" (split across varying
slots).
Debugging