Fix compositing mixing with framebuffer for web and ANGLE

[ghost]

Adds a #if defined(GRAPHICS_API_OPENGL_ES2) to clear the alpha channel on the end of the frame at the end of rlDrawRenderBatch() (R2894-R2900). It does that by:

1. Setting the clearColor alpha to 1;
2. Setting a colorMask to only consider the alpha from that clearColor;
3. Clearing it;
4. Restoring the colorMask to consider all values.

Removes background-color: black; from minshell.html (L35) and background: black; from shell.html (L78), since with the previous changes they are no longer needed. Those can still be set, but ClearBackground() will take precedence in defining the background color.

Tested these changes successfully on Firefox (102.11.0esr 64-bit) and Chrome (115.0.5790.110) on Linux (Linux Mint 21.1 x86_64).

@ypujante Could you please test/confirm if these changes fix the issue on ANGLE?

@raysan5 I tried my best to place that where I think is the end of the frame. If these proposed changes are accepted, could you please check if that's the ideal/correct place for them?

Fixes #3179.

Edit: added line marks.

[raysan5]

@ubkp I'm not sure about this change, does it consider the case that some applications could require a transparent background (some of my tools require it). Also, it should be probably called at EndDrawing() but calling it partially removes framebuffer cleaning control from user and it can derive in other unexpected issues, actually framebuffer cleaning was originally inside BeginDrawing() and it was problematic, so ClearBackground() was added.

[ghost]

@raysan5 Very excellent points!

Regarding transparency, I tested it on PLATFORM_DESKTOP with FLAG_WINDOW_TRANSPARENT and the change didn't break the window background transparency, neither its elements transparencies. But it would completely disable background transparency for anyone using OpenGL ES 2.0 (and break overlapping transparency of elements on OpenGL ES 2.0), which, indeed, I had not considered, my bad.

I'll try to find a better solution.