[3.x] Add disable_alpha render mode to shaders

[Calinou]

3.x version of #51711.

This can be used by multipass shaders to write to a texture's alpha channel without causing the material to become transparent.

Preview

Default render mode (GLES3)

disable_alpha render mode (GLES3)

Default render mode (GLES2)

disable_alpha render mode (GLES2)

[Lauson1ex]

@Calinou I made a test scene with a shader which renders the final colors to the RGB components of a Viewport, and depth to the alpha component. That Viewport's color and depth is then accessed by a 2D ColorRect elsewhere.

• When transparent_bg is disabled on the Viewport, reading the alpha from that Viewport always results in 0.0;
• When transparent_bg is enabled on the Viewport, reading the alpha from that Viewport works as expected, but RGB then always get premultiplied by the alpha.

transparent_bg = false, RGB components (after linear -> sRGB conversion):

transparent_bg = false, ALPHA component (depth, after screen-space -> view-space conversion):

(black screen because transparent_bg is disabled)

transparent_bg = true, RGB components (after linear -> sRGB conversion):

(black screen because ALPHA =< 0.0 due to depth being between 0.0 and -1.0 after screen-space to view-space transform)

transparent_bg = true, ALPHA component (depth, after screen-space -> view-space conversion):

And finally:
We can verify that RGB is being premultiplied by ALPHA by manually setting its value.

transparent_bg = true, ALPHA = 1.0, RGB components:

transparent_bg = true, ALPHA = 0.5, RGB components:

ALPHA is being correctly written to the Viewport, and it is being accessed by other shaders as expected. The problem is that the RGB components of a Viewport should not be premultiplied by ALPHA, especially when keep_3d_linear is enabled, which implies that the Viewport is being used as a data render target.

[clayjohn]

@Calinou to add to @Lauson1ex's comment above, you likely need to adjust the GLBlendFunc to GL_ONE somewhere.