glamor: add gl_PointSize for ES shaders

According to ESSL 1.0, if gl_PointSize is not defined explicitly,
then it will be undefined behaviour in shaders which use points.
So, define gl_PointSize explicitly for ES as 1.0, which is
desktop OpenGL default, so, shaders for ES and GL glamor will
behave the same.

As @anholt requested, define this only for minimal amount of shaders
(point and glyphbit ones), to make sure than performance will not
affected

Signed-off-by: Konstantin <ria.freelander@gmail.com>