pipeline: improve real_blend_enable checks

Since _cogl_pipeline_update_blend_enable() can sometimes show up quite
high in profiles; instead of calling
_cogl_pipeline_update_blend_enable() whenever we change pipeline state
that may affect blending we now just set a dirty flag and when we flush
a pipeline we check this dirty flag and lazily calculate whether blender
really needs to be enabled if it's set.

Since it turns out we were too optimistic in assuming most GL drivers
would recognize blending with ADD(src,0) is equivalent to disabling
GL_BLEND we now check this case ourselves so we can always explicitly
disable GL_BLEND if we know we don't need blending.

This introduces the idea of an 'unknown_color_alpha' boolean to the
pipeline flush code which is set whenever we can't guarantee that the
color attribute is opaque. For example this is set whenever a user
specifies a color attribute with 4 components when drawing a primitive.
This boolean needs to be cached along with every pipeline because
pipeline::real_blend_enabled depends on this and so we need to also call
_cogl_pipeline_update_blend_enable() if the status of this changes.

Incidentally with this patch we now no longer ever use
_cogl_pipeline_set_blend_enable() internally. For now the internal api
hasn't been removed though since we might want to consider re-purposing
it as a public api since it will now not conflict with our own internal
state tracking and could provide a more convenient way to disable
blending than setting a blend string.

Reviewed-by: Neil Roberts <neil@linux.intel.com>
conform: Add color attribute based blending test

This adds a test to make sure that if the same pipeline is used to draw
two primitives, one which doesn't need blending because it has an opaque
color associated, and another using a color attribute that requires
blending then Cogl should recognize that it needs to enable blending for
the second primitive even though the pipeline hasn't changed.

Reviewed-by: Neil Roberts <neil@linux.intel.com>
tests: Adds our first white-box unit test

This adds a white-box unit test that verifies that GL_BLEND is disabled
when drawing an opaque rectangle, enabled when drawing a transparent
rectangle and then disabled again when drawing a transparent rectangle
but with a blend string that effectively disables blending.

This shares the test utilities and launcher infrastructure we are using
for conformance tests so we get consistent reporting and so unit tests
will be run against a range of different drivers.

This adds a --enable-unit-tests configure option which is enabled by
default but if disabled will make all UNIT_TESTS() into static inline
functions that we should expect the compiler to discard since they won't
be referenced by anything.

Reviewed-by: Neil Roberts <neil@linux.intel.com>
units: remove test-bitmask and add UNIT_TEST instead

This moves the code in test-bitmask into a UNIT_TEST() directly in
cogl-bitmask.c which will now be run as a tests/unit/ test.

Reviewed-by: Neil Roberts <neil@linux.intel.com>
pipeline: only maintain breadcrumb for debug builds

This ensures we only add a static_breadcrumb pointer to every
CoglPipeline when build with debugging enabled. Since applications may
allocate a lot of pipelines we want to keep the basic size of pipelines
(ignoring optional sparse state) down to a minimum.

Reviewed-by: Neil Roberts <neil@linux.intel.com>