Window transparency & opacity

[Vinci10]

Description:

Fixes #181

This PR introduces window transparency (only background) and opacity (entire window: border and content) control to the Fyne framework. New methods are added to the fyne.Window interface:

Window Transparency:

Transparent() bool - Returns whether the window background is transparent
SetTransparent(bool) - Sets window transparency (must be called before Show() or ShowAndRun())

Window Opacity:

Opacity() float32 - Returns the current window opacity
SetOpacity(float32) - Sets window opacity (0.0 = fully transparent, 1.0 = fully opaque)

Implementation Details:

Desktop (GLFW): Full support for both transparency and opacity

• Uses glfw.TransparentFramebuffer hint for transparency
• Calls SetOpacity() on the viewport window for opacity control
• Clear color set to transparent (0,0,0,0) when transparency is enabled

Mobile, WebAssembly, Embedded (no-OS): Stubbed implementation (no-effect)

Changes Included:

• Added transparent and opacity fields to window structs across all drivers
• Updated painter to respect transparency setting when clearing
• Update demo tutorial showing both features
• Unit tests for GLFW implementation
• Tested on: Windows and Ubuntu

Checklist:

• Tests included.
• Lint and formatter run with no errors.
• Tests all pass.

Where applicable:

• Public APIs match existing style and have Since: line.
• Any breaking changes have a deprecation path or have been discussed.
• Check for binary size increases when importing new modules.

[coveralls]

coverage: 60.265% (-0.07%) from 60.337% — Vinci10:window_transparency into fyne-io:develop

[andydotxyz]

This certainly does not belong on fyne.Window as many OS do not support it (as you noted).
Do you know which of the desktop systems not tested should work? (macOS, BSD etc).

As noted in the original issue this will have impact on theme as well, because a transparent window doesn't help when the theme defines a fully opaque background colour.

I can see the use-case for needing transparent areas on a window - but what is the case for controlling opacity, in my experience that is set at the desktop level by the user / window manager.

[andydotxyz]

Noting as well that we have passed 2.8 deadline for features.