- Remove unused variables

-- MTLGraphics
---- color and ignoreScissor from MTLGraphics.nClear() method

-- MetalContext
---- resourceFactory, rttCleared, rttLoadAction, tripleBufferSemaphore, MAX_QUADS_IN_A_BATCH
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- Use matrix_identity_float4x4 variable instead of creating an identity matrix manually.
- matrix_identity_float4x4 variable provided by Apple simd library:
https://developer.apple.com/documentation/accelerate/matrix_identity_float4x4?language=objc
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- Remove a warning
-- MetalShader.m
Add [super dealloc] call to remove a warning: MetalShader.m:458:1: warning: method possibly missing a [super dealloc] call [-Wobjc-missing-super-calls]
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