RasterRenderer: Removed unused FAST_RSQRT patch

This code has been unused for 3 years now
RasterRenderer: Fixed Format
Screen/Ramp: Simplified bounds checks in ColorRampLookup() function
Screen/Ramp: Reduced array lookups
Screen/Ramp: Simplified ColorRampLookup()

One less indentation level for the complicated code
Screen/Ramp: Use pointer-based loop
Screen/Ramp: Extracted Interpolate() function
Screen/Ramp: Extracted ColorRamp::ToRGB8Color() method
Screen/Ramp: Added/Updated documentation
RasterRenderer: Added Documentation to MIX() function
RasterRenderer: Added Documentation to TerrainShading() function
RasterRenderer: Renamed Gmid/Gneighbor variables to center/neighbor
RasterMap: Added Documentation to PixelDistance() method
RasterRenderer: Added documentation to ScanMap() method
RasterRenderer: Moved documentation to the right code part

This comment was apparently corresponding to the integer cast of q
RasterRenderer: Renamed ColorTable() method to PrepareColorTable()

"ColorTable" is a noun, not a verb. Functions/Methods should describe what they
do, not what they are.
RasterRenderer: Improved PrepareColorTable() API documentation
(Weather)TerrainRenderer: Use nullptr instead of NULL