i965/blorp: Implement source clipping.

This patch modifies blorp blits (which are used for MSAA) to properly
account for clipping of source coordinates.  Previously, if we
detected the possibility of source clipping, we would fall back to the
blit meta-op, which doesn't support MSAA and is very slow for depth
and stencil buffers.

Fixes piglit tests
"EXT_framebuffer_multisample/clip-and-scissor-blit" on i965/Gen6+.

Also substantially speeds up the Humble Bundle V game "Psychonauts" on
Gen6+ (without this patch, the game's depth buffer blits use the slow
blit meta-op).

Reviewed-by: Carl Worth <cworth@cworth.org>

src/mesa/drivers/dri/i965/brw_blorp_blit.cpp

@@ -60,6 +60,9 @@ fixup_mirroring(bool &mirror, GLint &coord0, GLint &coord1)
  * For clarity, the nomenclature of this function assumes we are clipping and
  * scissoring the X coordinate; the exact same logic applies for Y
  * coordinates.
+ *
+ * Note: this function may also be used to account for clipping of source
+ * coordinates, by swapping the roles of src and dst.
  */
 static inline bool
 clip_or_scissor(bool mirror, GLint &src_x0, GLint &src_x1, GLint &dst_x0,
@@ -203,9 +206,14 @@ try_blorp_blit(struct intel_context *intel,
       return true;
    }
 
-   /* TODO: Clipping the source rectangle is not yet implemented. */
-   if (srcX0 < 0 || (GLuint) srcX1 > read_fb->Width) return false;
-   if (srcY0 < 0 || (GLuint) srcY1 > read_fb->Height) return false;
+   /* If the source rectangle needs to be clipped or scissored, do so. */
+   if (!(clip_or_scissor(mirror_x, dstX0, dstX1, srcX0, srcX1,
+                         0, read_fb->Width) &&
+         clip_or_scissor(mirror_y, dstY0, dstY1, srcY0, srcY1,
+                         0, read_fb->Height))) {
+      /* Everything got clipped/scissored away, so the blit was successful. */
+      return true;
+   }
 
    /* Get ready to blit.  This includes depth resolving the src and dst
     * buffers if necessary.