Support DontConvertToLinearOnRead & DontConvertToSRgbOnOnWrite in DrawFX

[gregsn]

I found a case where I had to convert a shader that was supposed to be used with the device in gamma color space. To make it work as expected I had to surround most of the pixel shader code with these calls:

    float4 col = Source.SampleLevel(Sampler0, tc, 0);
    col = ColorUtilityTemp.LinearToSRgb(col);

... col = ColorUtilityTemp.SRgbToLinear(col); return col;

What this does is that the pixel shader

• converts to linear space when sampling the _SRgb texture
• manually converts back And when writing to color buffer:
• converting to linear space manually
• converting to _SRgb on rasterization

It works but is quite an overhead.