Update pixman to 0.29.2
Improve OpenGLESv2 package detection.

Some OpenGLESv2 libraries may require extra include dirs (usually from
libGLESv2 dependencies) to build correctly. The pkg-config tools can handle
such situation when the .pc files are well constructed.

Using pkg-config we can also collect information about the CFLAGS that must be
set.

Now, the pkg_check_modules results are not used to feed find_path macros, they
are direclty exported instead. If pkg_check_modules fails to find the package,
find_path is used as fallback.

The FindOpenGLES2.cmake should be sent upstream in order to replace
FindGLES.cmake that does not use pkg-config at all.

Reviewed by: Caio Marcelo de Oliveira Filho
Reviewed by: Jesus Sanchez-Palencia
Fix use of OpenGLESv2 package detection results in PlatformNix.

If OpenGLESv2 is enabled, OPENGLES2_INCLUDE_DIRS, OPENGLES2_LIBRARIES and
OPENGLES2_DEFINITIONS must be added to build definitions.

Reviewed by: Caio Marcelo de Oliveira Filho
Reviewed by: Jesus Sanchez-Palencia