Update tests to not trigger undefined behaviour in framebuffer_fetch cases

Update tests to not trigger undefined behaviour in framebuffer_fetch cases

Some framebuffer_fetch cases had an integer format conversion which
resulted in overflow. The shader and reference texture generation have been
modified to take this in to account when creating values
in such way that it doesn't overflow the max range of the format.

Affects:

dEQP-GLES31.functional.shaders.framebuffer_fetch.framebuffer_format.*

Components: OpenGL

VK-GL-CTS issue: 2016

Change-Id: I686f6548cafee13dd2a0f564bb302d269ca6b0a0