glamor: OpenGL ES 2.0 fixes

Closed Yuriy Vasilev requested to merge uvas/xserver:gles20 into master

This patch solves two problems in glamor for OpenGL ES. The first bug resides in glamor_gradient.c, where glUniformMatrix3fv is used with argument transpose set to GL_TRUE. According to the Khronos OpenGL ES 2.0 pages transpose must be GL_FALSE.

Secondly, for 24 and 32 bit depth pictures xserver uses PICT_x8r8g8b8 and PICT_a8r8g8b8 formats, which must be backed with GL_BGRA format. It is present in OpenGL ES 2.0 only with GL_EXT_texture_format_BGRA8888 extension.

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