MAINT-3107 speculative changes to try to ameliorate rigged crasher on Intel HD with crappy default driver

NORSPEC-349 make render targets use original formats when resizing impostors and fix alpha for deferred impostor shader

NORSPEC-342 NORSPEC-352 make impostor alpha shader use vertex alpha to remedy unsightly blemishes, boxes, and the occasional extra wing.

NORSPEC-342 WIP MATBUG-347 make impostor rendering not contribute to depth for mostly-transparent buffer pixels, aka hair in ALM only

NORSPEC-314 fixed another mac-specific stupidity by splitting out a new srgb_mac version of the shader

MATBUG-315 remove unused uniforms from depth downsample frag shaders as they anger some shader compilers despite not being used

NORSPEC-285 NORSPEC-311 fix gamma correction of diffuse-only objects not matching the rest of the sRGB herd

NORSPEC-311 NORSPEC-312 remove unused shader and correct default for display gamma exposed in settings

NORSPEC-315 NORSPEC-316 NORSPEC-312 fix various haze and gamma correction issues, alpha-blend objects still missing haze underwater