WIP: OpenGL: use a single framebuffer instead of 1 per texture

[Zyko0]

What issue is this addressing?

No associated issue yet, related to: #2930 and #2953

What type of issue is this addressing?

Refactorization

What this PR does | solves

This PR uses a single framebuffer instead of creating a new one for each isolated texture for OpenGL.
This is a necessary step in order to introduce MRT, since a single framebuffer should be bound (itself containing the multiple destinations textures) for the MRT draw call.

Since a framebuffer represents a logical object, that is a group of buffers in openGL, it is not necessary to mark it with a size or create one for each texture, so it also should ease the internal logic hopefully!

Notes:

• Tested on windows desktop opengl + webgl
• No performance differences observed on my current project
• Setting this variable to 0, reproduces the previous behaviour of creating multiple framebuffers: https://github.com/hajimehoshi/ebiten/pull/2959/files#diff-2b469a3c6c95206c0ebd80925ccecd6ee726289212b1585da65c9043a3731b7aR324
• The DeleteFramebuffer function has been removed as there shouldn't be any need to delete one, since we want to control them to be exactly 2

Edit: It's actually more recommended to swap framebuffers, instead of changing its state every frame (its color attachments). Also, I realized manipulating attachments this way introduced some complexity, and MRT can be introduced without this by using a dedicated framebuffer.