Use a full screen triangle for mipmap calculations in mobile renderer #91480
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Fixes: #88786
Fixes: #88585
Fixes: #90818
Fixes: #91474
Regression introduced by #80311. What was happening here is that inside the shader we were expecting to render a full screen triangle. So we expect three points (0, 1, 2) to draw one triangle. Instead we pass an index array to draw a quad. So we draw the triangle with another triangle on top. BUT the triangle on top contains a vertex that is undefined because it results from an out of bound array access. Some hardware seems to just kill that triangle and work correctly (my intel iGPU does this). Other hardware sets that vertex to (0,0,0) (i.e. in the middle of the screen). Since the UVs of the second triangle don't make sense, you end up with that weird triangle floating at the top of the texture.