msaa: Generate proper error for operations prohibited on MSAA buffers.

From the GL 3.0 spec, section 4.3.3, in the documentation for
CopyPixels():

    "An INVALID_OPERATION error will be generated if the object bound
    to READ_FRAMEBUFFER_BINDING is framebuffer complete and the value
    of SAMPLE_BUFFERS is greater than zero."

The same applies to CopyTexImage...() and CopyTexSubImage...()
functions, since they are defined in terms of CopyPixels().

Previously we were generating an INVALID_FRAMEBUFFER_OPERATION error
in these cases.

Fixes piglit tests
"EXT_framebuffer_multisample/negative-{copypixels,copyteximage}".
msaa: Compute visual samples/sampleBuffers from all buffers.

This patch ensures that Visual.samples and Visual.sampleBuffers are
set correctly even in the case where there is no color buffer.
Previously, these values would retain their default value of 0 in this
circumstance, even if the depth or stencil buffer was multisampled.
i965/msaa: Control multisampling behaviour via the visual.

Previously, we used the number of samples in draw buffer 0 to
determine whether to set up the 3D pipeline for multisampling.  Using
the visual is cleaner, and has the benefit of working properly when
there is no color buffer.

Fixes all piglit tests "EXT_framebuffer_multisample/no-color" on Gen7.
On Gen6, the "depth-computed" variants of these tests still fail; this
will be addresed in a later patch.
i965: Set width, height, and tiling properly for null render targets.

The HW docs say that the width and height of null render targets need
to match the width and height of the corresponding depth and/or
stencil buffers, and that they need to be marked as Y-tiled.  Although
leaving these values at 0 doesn't seem to cause any ill effects, it
seems wise to follow the documented requirements.
i965/msaa: Work around problems with null render targets on Gen6.

On Gen6, multisampled null render targets don't seem to work
properly--they cause the GPU to hang.  So, as a workaround, we render
into a dummy color buffer.

Fortunately this situation (multisampled rendering without a color
buffer) is rare, and we don't have to waste too much memory, because
we can give the workaround buffer a very small pitch.

Fixes piglit test "EXT_framebuffer_multisample/no-color {2,4}
depth-computed *" on Gen6.
i965/blorp: Simplify check that src/dst width/height match.

When checking that the source and destination dimensions match, we
don't need to store the width and height in variables; doing so just
risks confusion since right after the check, we do clipping and
scissoring, which may alter the width and height.

No functional change.
i965/blorp: Fixup scissoring of blits to window system buffers.

This patch modifies the order of operations in the blorp engine so
that clipping and scissoring are performed before adjusting the
coordinates to account for the difference in origin convention between
window system buffers and framebuffer objects.  Previously, we would
do clipping and scissoring after adjusting for origin conventions, so
we would get scissoring wrong in window system buffers.

Fixes Piglit test "fbo-scissor-blit window".
i965/blorp: Don't fall back to swrast when miptrees absent.

Previously, the blorp engine would fall back to swrast if the source
or destination of a blit had no associated miptree.  This was
unnecessary, since _mesa_BlitFramebufferEXT() already takes care of
making the blit silently succeed if there are no buffers bound, so the
fallback paths could never actually happen in practice.

Removing these fallback paths will simplify the implementation of
correct DrawBuffers support in blorp.
i965/blorp: Rearrange order of blit validation and preparation steps.

This patch rearranges the order of steps performed by a blorp blit
from this:

- Sync up state of window system buffers.
- Find buffers.
- Find miptrees.
- Make sure buffer formats match.
- Handle mirroring.
- Make sure width and height match.
- Handle clipping/scissoring.
- Account for window system origin conventions.
- Do depth resolves, if applicable.
- Do the blit.
- Record the need for a future HiZ resolve, if applicable.

To this:

- Sync up state of window system buffers.
- Handle mirroring.
- Make sure width and height match.
- Handle clipping/scissoring.
- Account for window system origin conventions.
- Find buffers.
- Make sure buffer formats match.
- Find miptrees.
- Do depth resolves, if applicable.
- Do the blit.
- Record the need for a future HiZ resolve, if applicable.

The steps are the same, but they are now performed in an order that
will make it possible to implement correct DrawBuffers support.  Note
that the last four steps are now in a separate function
(do_blorp_blit), since they will need to be executed repeatedly when
DrawBuffers support is added.
i965/blorp: Handle DrawBuffers properly.

When the client program uses glDrawBuffer() or glDrawBuffers() to
select more than one color buffer for drawing into, and then performs
a blit, we need to blit into every single enabled draw buffer.

+2 oglconforms.
i965/msaa: Remove comment about falsely claiming to support MSAA.

Gen6+ hardware now supports MSAA properly.