Fix Gaussian Masking Logic & Eliminate Code Duplication

[Wang-Chbo]

#15 raised a very interesting question that remains unanswered. However, in reality, the function name is quite misleading. The final code significantly relaxes the positional requirements for Gaussians, which is necessary: Imagine a scenario where the center of a Gaussian is not within the current view frustum, but the Gaaussian's diameter is large enough to affect many pixels within the current view frustum. In this case, we should not cull this Gaussian from rendering.
Additionally, I noticed that the calculations in Lines 149-152 of in_frustum duplicate code found elsewhere, such as Lines 196-200 in forward.cu. Therefore, I recommend deleting Lines 149-151 in in_frustum.

diff-gaussian-rasterization/cuda_rasterizer/auxiliary.h

Lines 139 to 165 in 59f5f77

	__forceinline__ __device__ bool in_frustum(int idx,
	const float* orig_points,
	const float* viewmatrix,
	const float* projmatrix,
	bool prefiltered,
	float3& p_view)
	{
	float3 p_orig = { orig_points[3 * idx], orig_points[3 * idx + 1], orig_points[3 * idx + 2] };
	// Bring points to screen space
	float4 p_hom = transformPoint4x4(p_orig, projmatrix);
	float p_w = 1.0f / (p_hom.w + 0.0000001f);
	float3 p_proj = { p_hom.x * p_w, p_hom.y * p_w, p_hom.z * p_w };
	p_view = transformPoint4x3(p_orig, viewmatrix);
	if (p_view.z <= 0.2f)// || ((p_proj.x < -1.3 || p_proj.x > 1.3 || p_proj.y < -1.3 || p_proj.y > 1.3)))
	{
	if (prefiltered)
	{
	printf("Point is filtered although prefiltered is set. This shouldn't happen!");
	__trap();
	}
	return false;
	}
	return true;
	}

diff-gaussian-rasterization/cuda_rasterizer/forward.cu

Lines 196 to 200 in 59f5f77

	// Transform point by projecting
	float3 p_orig = { orig_points[3 * idx], orig_points[3 * idx + 1], orig_points[3 * idx + 2] };
	float4 p_hom = transformPoint4x4(p_orig, projmatrix);
	float p_w = 1.0f / (p_hom.w + 0.0000001f);
	float3 p_proj = { p_hom.x * p_w, p_hom.y * p_w, p_hom.z * p_w };