Apply sRGB conversion to reference in sRGB framebuffer fetch test

Apply sRGB conversion to reference in sRGB framebuffer fetch test

Framebuffer fetch returns linear values when the framebuffer format
is sRGB and GL_FRAMEBUFFER_SRGB is enabled (default). This conversion
was not applied when generating reference textures for framebuffer
fetch tests. glReadPixels() does not apply any conversion, so the
final reference value must also be converted back to sRGB.

Affects: dEQP-GLES31.functional.shaders.framebuffer_fetch.
framebuffer_format.srgb8_alpha8

Components: AOSP