gallium: Add a dumb drm/kms winsys backed swrast provider

Add a new winsys and target that can be used with a dri2 state tracker
and loader instead of drisw. This allows to use gbm as a dri2/image
loader and avoid the extra copy from the backbuffer to the shadow
frontbuffer.

The new driver is called "kms_swrast", and is loaded by gbm as a
fallback, because it is only useful with the gbm platform (as no buffer
sharing is possible)

To force select the driver set the environment variable
GBM_ALWAYS_SOFTWARE

[Emil Velikov]
 - Rebase on top of gallium megadriver.
 - s/text/test/ in configure.ac (Spotted by Andreas Pokorny).
 - Add scons support for winsys/sw/kms-dri and fix the build.
 - Provide separate DriverAPI, due to different InitScreen hook.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>

docs/relnotes/10.3.html

@@ -59,6 +59,8 @@ Note: some of the new features are only available with certain drivers.
 <li>GL_ARB_fragment_layer_viewport on nv50, nvc0, llvmpipe, r600</li>
 <li>GL_AMD_vertex_shader_viewport_index on i965/gen7+, r600</li>
 <li>GL_ARB_clear_texture on i965</li>
+<li>A new software rasterizer driver (kms_swrast_dri.so) that works with
+DRM drivers that don't have a full-fledged GEM (such as qxl or simpledrm)</li>
 </ul>