glamor: fix for 8bit fonts and PointSize on GLES 2

Closed Konstantin requested to merge rilian-la-te/xserver:glamor-es-fixes-tests into master

This is a second part of GLES2 fixes, which fixed font problems on ES2, if you do not have GLES3.

This contains some controversal change, which leads to correct rendering, but have a side effect by allowing formats not readable on GLES2 to be used as glTexImage2D, if we do not want to actually read them by glReadPixels.

Fixes #1362
Fixes #1411

Request-For-Review: @ofourdan @anholt @p12tic @keithp

Edited by Konstantin

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