Fix issues related to sRGB correct rendering

[ensisoft]

• The color picker used by the editor is likely giving us back sRGB encoded color values. These need to be converted into linear before being used for any computation or passed onto shaders.

[ensisoft]

• Make sure to convert any "color" (vec4) uniform from sRGB to linear.
• Change default texture color space from Linear to sRGB
• Make sure the color buffer writes works properly (in case the color buffer is sRGB)
• Remove the useless gamma value from materials
• Clear value in glClearColor
• Read values in glReadPixels (screenshot functionality)