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, 15 insertions, 0 deletions

diff --git a/src/cairo-image-surface-private.h b/src/cairo-image-surface-private.h
index 227a447c3..2777e2f3d 100644
--- a/src/cairo-image-surface-private.h
+++ b/src/cairo-image-surface-private.h
@@ -74,6 +74,7 @@ typedef struct _cairo_image_source {
 } cairo_image_source_t;
 
 cairo_private extern const cairo_surface_backend_t _cairo_image_surface_backend;
+cairo_private extern const cairo_surface_backend_t _cairo_image_source_backend;
 
 cairo_private const cairo_compositor_t *
 _cairo_image_mask_compositor_get (void);
@@ -163,6 +164,20 @@ _cairo_surface_is_image (const cairo_surface_t *surface)
     return surface->backend == &_cairo_image_surface_backend;
 }
 
+/**
+ * _cairo_surface_is_image_source:
+ * @surface: a #cairo_surface_t
+ *
+ * Checks if a surface is an #cairo_image_source_t
+ *
+ * Return value: %TRUE if the surface is an image source
+ **/
+static inline cairo_bool_t
+_cairo_surface_is_image_source (const cairo_surface_t *surface)
+{
+    return surface->backend == &_cairo_image_source_backend;
+}
+
 CAIRO_END_DECLS
 
 #endif /* CAIRO_IMAGE_SURFACE_PRIVATE_H */