Fix shader issues

[riccardobl]

Issues found by testing the shaders under angle GLES3

[gemini-code-assist]

Code Review

This pull request updates several GLSL shaders to prevent implicit type conversion errors by explicitly casting integers to floats, and initializes newly added fields in the Light and PBRSurface structures. The review feedback highlights a few remaining implicit conversion issues in Downsample.frag (lines 27 and 29), suggests simplifying a redundant ternary operation in the calculation of surface.NdotV, recommends comparing the integer layer directly with integer literals instead of casting it to a float, and points out a duplicate assignment of surface.alpha.