fix(flame_3d): Fix alpha blend factors for transparent background compositing

[agnath18K]

Description

Fixes #3811

The alphaBlend path in GraphicsDevice.begin() sets sourceAlphaBlendFactor: oneMinusSourceAlpha, which zeroes the alpha channel for opaque fragments:

result_alpha = src_alpha * (1 - src_alpha) = 1.0 * 0.0 = 0

The GPU texture has valid RGB but zero alpha, so it becomes invisible when composited onto the Flutter canvas via srcOver.

This swaps the factors to match the standard premultiplied "over" operator:

• sourceAlphaBlendFactor: one — preserves source alpha
• destinationAlphaBlendFactor: oneMinusSourceAlpha — blends with destination

Checklist

• I have followed the Contributor Guide when preparing my PR.
• I have updated/added tests for ALL new/updated/fixed functionality.
• I have updated/added relevant documentation in docs and added dartdoc comments with ///.
• I have updated/added relevant examples in examples or docs.

The blend equation change is purely in the GPU pipeline — not feasible to unit test without a real GPU context. Verified manually by rendering models with clearColor: 0x00000000 and compositing over Flutter widgets.

Breaking Change?

• Yes, this PR is a breaking change.
• No, this PR is not a breaking change.

[Copilot]

Pull request overview

This PR attempts to fix transparent background compositing in flame_3d by correcting the alpha blend factors used in the GPU pipeline. The issue was that the alpha channel was being zeroed out for opaque fragments due to incorrect blend factor configuration, making rendered 3D scenes invisible when composited over Flutter widgets.

Changes:

• Modified alpha blend factor configuration in GraphicsDevice.begin() to use premultiplied alpha "over" operator

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[Copilot]

The fix is incomplete. According to the PR description and issue #3811, the correct premultiplied "over" operator requires setting BOTH source and destination alpha blend factors:

• sourceAlphaBlendFactor: one (to preserve source alpha)
• destinationAlphaBlendFactor: oneMinusSourceAlpha (to blend with destination)

The current change only sets destinationAlphaBlendFactor without setting sourceAlphaBlendFactor. This means the source alpha blend factor is not configured at all, which may result in undefined or default behavior that doesn't match the intended fix. The ColorBlendEquation constructor should include both parameters.

Suggested change

	gpu.ColorBlendEquation(
	gpu.ColorBlendEquation(
	sourceAlphaBlendFactor: gpu.BlendFactor.one,