Remove wrongly repeated words in comments

Clean up misrepetitions ('if if', 'the the' etc) found throughout the
comments. This has been done manually, after grepping
case-insensitively for duplicate if, is, the, then, do, for, an,
plus a few other typos corrected in fly-by

v2:
    * proper commit message and non-joke title;
    * replace two 'as is' followed by 'is' to 'as-is'.
v3:
    * 'a integer' => 'an integer' and similar (originally spotted by
      Jason Ekstrand, I fixed a few other similar ones while at it)

Signed-off-by: Giuseppe Bilotta <giuseppe.bilotta@gmail.com>
Reviewed-by: Chad Versace <chad.versace@intel.com>

src/gallium/docs/source/context.rst

@@ -234,7 +234,7 @@ include several layers), this surface need not be bound to the framebuffer.
 
 ``clear_depth_stencil`` clears a single depth, stencil or depth/stencil surface
 with the specified depth and stencil values (for combined depth/stencil buffers,
-is is also possible to only clear one or the other part). While it is only
+it is also possible to only clear one or the other part). While it is only
 possible to clear one surface at a time (which can include several layers),
 this surface need not be bound to the framebuffer.