backends: Adds a new Cogl based backend

This adds a new GPU accelerated backend for Cairo based on the Cogl 3D
graphics API.

This backend aims to support Cairo in a way that translates as naturally
as possible to using a GPU, it does not strive to compete with the
anti-aliasing quality of the image backend if it can't be done
efficiently using the GPU - raw performance isn't the only metric of
concern, so is power usage.

As an overview of how the backend works:
- fills are handled by tessellating paths into triangles
- the backend has an extra fill_rectangle drawing operation so we have
  a fast-path for drawing rectangles which are so common.
- strokes are also tessellated into triangles.
- stroke and fill tessellations are cached to avoid the cpu overhead
  of tessellation and cost of upload given that its common for apps to
  re-draw the same path multiple times. The tessellations can survive
  translations and rotations increasing the probability that they can be
  re-used.
- sources and masks are handled using multi-texturing.
- clipping is handled with a scissor and the stencil buffer which
  we're careful to only update when they really change.
- linear gradients are rendered to a 1d texture using a triangle
  strip + interpolating color attributes. All cairo extend modes
  are handled by corresponding texture sampler wrap modes without
  needing programmable fragment processing.
- antialiasing should be handled using Cogl's multisampling API

XXX: This is a work in progress!!
TODO:
- handle at least basic radial gradients (No need to handle full
  pdf semantics, since css, svg and canvas only allow radial gradients
  defined as one circle + a point that must lie within the first
  circle.) - currently we fall back to pixman for radial gradients.
- support glyph rendering with a decent glyph cache design. The
  current plan is a per scaled-font growable cache texture + a
  scratch cache for one-shot/short-lived glyphs.
- decide how to handle npot textures when lacking hardware support.
  Current plan is to add a transparent border to npot textures and use
  CLAMP_TO_EDGE for the default EXTEND_NONE semantics. For anything else
  we can allocate a shadow npot texture and scale the original to fit
  that so we can map extend modes to texture sampler modes.

1 files changed, 24 insertions, 12 deletions

diff --git a/src/cairo-default-context.c b/src/cairo-default-context.c
index 98cc6684d..b58a76614 100644
--- a/src/cairo-default-context.c
+++ b/src/cairo-default-context.c
@@ -63,11 +63,9 @@ _cairo_default_context_reset_static_data (void)
     _freed_pool_reset (&context_pool);
 }
 
-static void
-_cairo_default_context_destroy (void *abstract_cr)
+void
+_cairo_default_context_fini (cairo_default_context_t *cr)
 {
-    cairo_default_context_t *cr = abstract_cr;
-
     while (cr->gstate != &cr->gstate_tail[0]) {
 	if (_cairo_gstate_restore (&cr->gstate, &cr->gstate_freelist))
 	    break;
@@ -84,6 +82,14 @@ _cairo_default_context_destroy (void *abstract_cr)
     _cairo_path_fixed_fini (cr->path);
 
     _cairo_fini (&cr->base);
+}
+
+static void
+_cairo_default_context_destroy (void *abstract_cr)
+{
+    cairo_default_context_t *cr = abstract_cr;
+
+    _cairo_default_context_fini (cr);
 
     /* mark the context as invalid to protect against misuse */
     cr->base.status = CAIRO_STATUS_NULL_POINTER;
@@ -1381,6 +1387,19 @@ static const cairo_backend_t _cairo_default_context_backend = {
     _cairo_default_context_show_page,
 };
 
+cairo_status_t
+_cairo_default_context_init (cairo_default_context_t *cr, void *target)
+{
+    _cairo_init (&cr->base, &_cairo_default_context_backend);
+    _cairo_path_fixed_init (cr->path);
+
+    cr->gstate = &cr->gstate_tail[0];
+    cr->gstate_freelist = &cr->gstate_tail[1];
+    cr->gstate_tail[1].next = NULL;
+
+    return _cairo_gstate_init (cr->gstate, target);
+}
+
 cairo_t *
 _cairo_default_context_create (void *target)
 {
@@ -1394,14 +1413,7 @@ _cairo_default_context_create (void *target)
 	    return _cairo_create_in_error (_cairo_error (CAIRO_STATUS_NO_MEMORY));
     }
 
-    _cairo_init (&cr->base, &_cairo_default_context_backend);
-    _cairo_path_fixed_init (cr->path);
-
-    cr->gstate = &cr->gstate_tail[0];
-    cr->gstate_freelist = &cr->gstate_tail[1];
-    cr->gstate_tail[1].next = NULL;
-
-    status = _cairo_gstate_init (cr->gstate, target);
+    status = _cairo_default_context_init (cr, target);
     if (unlikely (status)) {
 	_freed_pool_put (&context_pool, cr);
 	return _cairo_create_in_error (status);