Docs updates.

        { SkXfermode::kXor_Mode,     GL_ONE_MINUS_DST_ALPHA,  GL_ONE_MINUS_SRC_ALPHA }

};

// This array contains the swapped version of each SkXfermode. For instance

// this array's SrcOver blending mode is actually DstOver. You can refer to

// createLayer() for more information on the purpose of this array.

static const Blender gBlendsSwap[] = {

        { SkXfermode::kClear_Mode,   GL_ZERO,                 GL_ZERO },

        { SkXfermode::kSrc_Mode,     GL_ZERO,                 GL_ONE },

 * This implementation relies on the frame buffer being at least RGBA 8888. When

 * a layer is created, only a texture is created, not an FBO. The content of the

 * frame buffer contained within the layer's bounds is copied into this texture

 * using glCopyTexImage2D(). The layer's region is then cleared in the frame

 * using glCopyTexImage2D(). The layer's region is then cleared(1) in the frame

 * buffer and drawing continues as normal. This technique therefore treats the

 * frame buffer as a scratch buffer for the layers.

 *

 * Because glCopyTexImage2D() can be slow, an alternative implementation might

 * be use to draw a single clipped layer. The implementation described above

 * is correct in every case.

 *

 * (1) The frame buffer is actually not cleared right away. To allow the GPU

 *     to potentially optimize series of calls to glCopyTexImage2D, the frame

 *     buffer is left untouched until the first drawing operation. Only when

 *     something actually gets drawn are the layers regions cleared.

 */

bool OpenGLRenderer::createLayer(sp<Snapshot> snapshot, float left, float top,

        float right, float bottom, int alpha, SkXfermode::Mode mode,int flags) {

    bool clipTransformed(const Rect& r, SkRegion::Op op = SkRegion::kIntersect_Op) {

        bool clipped = false;

        // NOTE: The unimplemented operations require support for regions

        // Supporting regions would require using a stencil buffer instead

        // of the scissor. The stencil buffer itself is not too expensive

        // (memory cost excluded) but on fillrate limited devices, managing

        // the stencil might have a negative impact on the framerate.

        switch (op) {

            case SkRegion::kDifference_Op:

                break;